485BPOS

As filed with the U.S. Securities and Exchange Commission on November 6, 2014

Securities Act File No. 002-57354

Investment Company Act File No. 811-02688

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933	x
Pre-Effective Amendment No.	¨
Post-Effective Amendment No. 55	x
and/or
REGISTRATION STATEMENT
UNDER
THE INVESTMENT COMPANY ACT OF 1940	x

Amendment No. 53

(Check appropriate box or boxes)

BLACKROCK MUNICIPAL BOND

FUND, INC.

(Exact Name of Registrant as Specified in Charter)

100 Bellevue Parkway

Wilmington, Delaware 19809

(Address of Principal Executive Office)

Registrant’s Telephone Number, including Area Code (800) 441-7762

John M. Perlowski

BLACKROCK MUNICIPAL BOND FUND, INC.

55 East 52^nd Street, New York, New York 10055

(Name and Address of Agent for Service)

Copies to:

Counsel for the Fund:
Margery K. Neale, Esq.
Willkie Farr & Gallagher LLP
787 Seventh Avenue
New York, New York 10019-6099 Benjamin Archibald, Esq.
BlackRock Advisors, LLC
55 East 52^nd Street
New York, New York 10055

Continuous

(Approximate Date of Proposed Offering)

It is proposed that this filing will become effective:

x immediately upon filing pursuant to paragraph (b)

¨ on (date) pursuant to paragraph (b)

¨ 60 days after filing pursuant to paragraph (a)(1)

¨ on (date) pursuant to paragraph (a)(1)

¨ 75 days after filing pursuant to paragraph (a)(2)

¨ on (date) pursuant to paragraph (a)(2) of Rule 485.

If appropriate, check the following box:

¨ This post-effective amendment designates a new effective date for a previously filed post-effective amendment.

Title of Securities Being Registered: Shares of Common Stock, par value, $0.10 per share.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933 and the Investment Company Act of 1940, the Registrant certifies that it meets all the requirements for the effectiveness of this Post-Effective Amendment to its Registration Statement pursuant to
Rule 485(b) under the Securities Act of 1933 and has duly caused this Post-Effective Amendment to its Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of New York and the State of New York, on November 6, 2014.

	BlackRock Municipal Bond Fund, Inc.
	(Registrant)
By:	/s/ JOHN M. PERLOWSKI
	(John M. Perlowski, President and Chief Executive Officer) (Principal Executive Officer)

Pursuant to the requirements of the Securities Act of 1933, this Post-Effective Amendment to its Registration Statement has been signed below by the following persons in the capacities and on the dates indicated:

Signature	Title	Date
/s/ JOHN M. PERLOWSKI	President and Chief Executive Officer	November 6, 2014
John M. Perlowski	(Principal Executive Officer)
/s/ NEAL J. ANDREWS	Chief Financial Officer (Principal	November 6, 2014
Neal J. Andrews	Financial and Accounting Officer)
JAMES H. BODURTHA*	Director
(James H. Bodurtha)
BRUCE R. BOND*	Director
(Bruce R. Bond)
DONALD W. BURTON*	Director
(Donald W. Burton)
STUART E. EIZENSTAT*	Director
(Stuart E. Eizenstat)
KENNETH A. FROOT*	Director
(Kenneth A. Froot)
ROBERT M. HERNANDEZ*	Director
(Robert M. Hernandez)

- 3 -

JOHN F. O’BRIEN*	Director
(John F. O’Brien)
ROBERTA COOPER RAMO*	Director
(Roberta Cooper Ramo)
DAVID H. WALSH*	Director
(David H. Walsh)
FRED G. WEISS*	Director
(Fred G. Weiss)
PAUL L. AUDET*	Director
(Paul L. Audet)
LAURENCE D. FINK*	Director
(Laurence D. Fink)
HENRY GABBAY*	Director
(Henry Gabbay)

*By:	/s/ BENJAMIN ARCHIBALD	November 6, 2014
	Benjamin Archibald (Attorney-in-Fact)

- 4 -

EXHIBIT INDEX

Index No.	Description of Exhibit
EX-101.INS	XBRL Instance Document
EX-101.SCH	XBRL Taxonomy Extension Schema Document
EX-101.CAL	XBRL Taxonomy Extension Calculation Linkbase
EX-101.DEF	XBRL Taxonomy Extension Definition Linkbase
EX-101.LAB	XBRL Taxonomy Extension Labels Linkbase
EX-101.PRE	XBRL Taxonomy Extension Presentation Linkbase

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	BLACKROCK MUNICIPAL BOND FUND, INC.
Prospectus Date	rr_ProspectusDate	Oct. 28, 2014
BlackRock | BlackRock Short-Term Municipal Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Overview Key Facts About BlackRock Short-Term Municipal Fund
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The investment objective of BlackRock Short-Term Municipal Fund (the “Short-Term Fund” or the “Fund”) is to provide shareholders with as high a level of income exempt from Federal income taxes as is consistent with the investment policies of the Fund.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold BlackRock Shares of the Fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover:
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 56% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	56.00%
Expense Example [Heading]	rr_ExpenseExampleHeading	Example:
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies of the Fund
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the Short-Term Fund seeks to achieve its objective by investing at least 80% of its assets in municipal bonds. Municipal bonds include debt obligations issued by or on behalf of a governmental entity or other qualifying issuer that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax). Municipal bonds may be obligations of a variety of issuers, including governmental entities or other qualifying issuers. Issuers may be states, territories and possessions of the United States and the District of Columbia and their political subdivisions, agencies and instrumentalities. Municipal bonds also include short-term tax-exempt obligations like municipal notes and variable rate demand obligations. The Short-Term Fund invests primarily in investment grade municipal bonds or municipal notes, including variable rate demand obligations. Investment grade bonds are securities which are rated in the four highest categories by at least one of the major rating agencies or determined by the management team to be of similar quality. Generally, the higher the rating of a bond, the higher the likelihood that interest and principal payments will be made on time. Municipal notes are shorter-term municipal debt obligations that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax) and that have a maturity that is generally one year or less. The Fund will maintain a dollar weighted maturity of no more than two years. Because of their shorter maturities, the Fund's investments will not usually be as sensitive to changes in prevailing interest rates as are long-term municipal bonds. Fluctuations in interest rates on short-term municipal bonds may, however, vary more widely from time to time than those on long-term municipal bonds.
Risk [Heading]	rr_RiskHeading	Principal Risks of Investing in the Fund
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund. Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things. Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund's investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund's investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund's net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the "full faith and credit" of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund's performance. Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make principal and interest payments when due. Changes in an issuer's credit rating or the market's perception of an issuer's creditworthiness may also affect the value of the Fund's investment in that issuer. The degree of credit risk depends on the issuer's financial condition and on the terms of the securities. Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall. Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields. Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund's portfolio will be magnified when the Fund uses leverage. Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund's investment in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund's principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquid investments may become illiquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions. Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money. Municipal Securities Risks — Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include: General Obligation Bonds Risks — Timely payments depend on the issuer's credit quality, ability to raise tax revenues and ability to maintain an adequate tax base. Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source. Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its faith, credit and taxing power for repayment. Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality. Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money. Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property. Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers' bond counsel and, in the case of derivative securities, sponsors' counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities. Variable Rate Demand Obligations Risk — Variable rate demand obligations are floating rate securities that combine an interest in a long-term municipal bond with a right to demand payment before maturity from a bank or other financial institution. If the bank or financial institution is unable to pay, the Fund may lose money.
Risk Lose Money [Text]	rr_RiskLoseMoney	You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the S&P® Municipal Bond Index and the S&P® Limited Maturity Municipal Bond Index, which are relevant to the Fund because they have characteristics similar to the Fund’s investment strategies. The returns for BlackRock Shares prior to October 2, 2006, the commencement of operations of BlackRock Shares, are based upon performance of the Fund’s Institutional Shares, as adjusted to reflect class-specific fees applicable to BlackRock Shares. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com/funds or can be obtained by phone at (800) 882-0052.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	(800) 882-0052
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.blackrock.com/funds
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	As with all such investments, past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading]	rr_BarChartHeading	BlackRock Shares ANNUAL TOTAL RETURNS BlackRock Short-Term Municipal Fund As of 12/31
Bar Chart Does Not Reflect Sales Loads [Text]	rr_BarChartDoesNotReflectSalesLoads	Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown.
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	During the ten-year period shown in the bar chart, the highest return for a quarter was 1.65% (quarter ended December 31, 2008) and the lowest return for a quarter was -0.43% (quarter ended June 30, 2013). The year-to-date return as of September 30, 2014 was 0.35%.
Performance Table Heading	rr_PerformanceTableHeading	As of 12/31/13 Average Annual Total Returns
Performance Table Does Reflect Sales Loads	rr_PerformanceTableDoesReflectSalesLoads	However, the table includes all applicable fees and sales charges.
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
BlackRock | BlackRock Short-Term Municipal Fund | BlackRock Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none
Management Fee	rr_ManagementFeesOverAssets	0.33%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.06%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.39%
1 Year	rr_ExpenseExampleYear01	$ 40
3 Years	rr_ExpenseExampleYear03	125
5 Years	rr_ExpenseExampleYear05	219
10 Years	rr_ExpenseExampleYear10	$ 493
2004	rr_AnnualReturn2004	0.72%
2005	rr_AnnualReturn2005	1.40%
2006	rr_AnnualReturn2006	2.85%
2007	rr_AnnualReturn2007	4.04%
2008	rr_AnnualReturn2008	3.75%
2009	rr_AnnualReturn2009	3.68%
2010	rr_AnnualReturn2010	0.65%
2011	rr_AnnualReturn2011	1.65%
2012	rr_AnnualReturn2012	0.69%
2013	rr_AnnualReturn2013	0.48%
Year to Date Return, Label	rr_YearToDateReturnLabel	year-to-date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2014
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.35%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2008
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.65%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Jun. 30, 2013
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(0.43%)
1 Year	rr_AverageAnnualReturnYear01	0.48%
5 Years	rr_AverageAnnualReturnYear05	1.42%
10 Years	rr_AverageAnnualReturnYear10	1.98%
BlackRock | BlackRock Short-Term Municipal Fund | Return After Taxes on Distributions | BlackRock Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	0.48%
5 Years	rr_AverageAnnualReturnYear05	1.42%
10 Years	rr_AverageAnnualReturnYear10	1.98%
BlackRock | BlackRock Short-Term Municipal Fund | Return After Taxes on Distributions and Sale of Fund Shares | BlackRock Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	0.53%
5 Years	rr_AverageAnnualReturnYear05	1.38%
10 Years	rr_AverageAnnualReturnYear10	1.97%
BlackRock | BlackRock Short-Term Municipal Fund | S&P ® Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(2.55%)
5 Years	rr_AverageAnnualReturnYear05	6.34%
10 Years	rr_AverageAnnualReturnYear10	4.33%
BlackRock | BlackRock Short-Term Municipal Fund | S&P ® Limited Maturity Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	1.02%
5 Years	rr_AverageAnnualReturnYear05	2.18%
10 Years	rr_AverageAnnualReturnYear10	2.68%

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	BLACKROCK MUNICIPAL BOND FUND, INC.
Prospectus Date	rr_ProspectusDate	Oct. 28, 2014
Investor A1 | BlackRock Short-Term Municipal Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Overview Key Facts About BlackRock Short-Term Municipal Fund
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The investment objective of BlackRock Short-Term Municipal Fund (the “Short-Term Fund” or the “Fund”) is to provide shareholders with as high a level of income exempt from Federal income taxes as is consistent with the investment policies of the Fund.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Investor A1 Shares of the Fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	November 1, 2015
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover:
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 56% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	56.00%
Expense Example [Heading]	rr_ExpenseExampleHeading	Example:
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies of the Fund
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the Short-Term Fund seeks to achieve its objective by investing at least 80% of its assets in municipal bonds. Municipal bonds include debt obligations issued by or on behalf of a governmental entity or other qualifying issuer that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax). Municipal bonds may be obligations of a variety of issuers, including governmental entities or other qualifying issuers. Issuers may be states, territories and possessions of the United States and the District of Columbia and their political subdivisions, agencies and instrumentalities. Municipal bonds also include short-term tax-exempt obligations like municipal notes and variable rate demand obligations. The Short-Term Fund invests primarily in investment grade municipal bonds or municipal notes, including variable rate demand obligations. Investment grade bonds are securities which are rated in the four highest categories by at least one of the major rating agencies or determined by the management team to be of similar quality. Generally, the higher the rating of a bond, the higher the likelihood that interest and principal payments will be made on time. Municipal notes are shorter-term municipal debt obligations that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax) and that have a maturity that is generally one year or less. The Fund will maintain a dollar weighted maturity of no more than two years. Because of their shorter maturities, the Fund's investments will not usually be as sensitive to changes in prevailing interest rates as are long-term municipal bonds. Fluctuations in interest rates on short-term municipal bonds may, however, vary more widely from time to time than those on long-term municipal bonds.
Risk [Heading]	rr_RiskHeading	Principal Risks of Investing in the Fund
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund. Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things. Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund's investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund's investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund's net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the "full faith and credit" of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund's performance. Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make principal and interest payments when due. Changes in an issuer's credit rating or the market's perception of an issuer's creditworthiness may also affect the value of the Fund's investment in that issuer. The degree of credit risk depends on the issuer's financial condition and on the terms of the securities. Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall. Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields. Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund's portfolio will be magnified when the Fund uses leverage. Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund's investment in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund's principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquid investments may become illiquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions. Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money. Municipal Securities Risks — Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include: General Obligation Bonds Risks — Timely payments depend on the issuer's credit quality, ability to raise tax revenues and ability to maintain an adequate tax base. Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source. Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its faith, credit and taxing power for repayment. Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality. Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money. Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property. Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers' bond counsel and, in the case of derivative securities, sponsors' counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities. Variable Rate Demand Obligations Risk — Variable rate demand obligations are floating rate securities that combine an interest in a long-term municipal bond with a right to demand payment before maturity from a bank or other financial institution. If the bank or financial institution is unable to pay, the Fund may lose money.
Risk Lose Money [Text]	rr_RiskLoseMoney	You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the S&P® Municipal Bond Index and the S&P® Limited Maturity Municipal Bond Index, which are relevant to the Fund because they have characteristics similar to the Fund’s investment strategies. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com/funds or can be obtained by phone at (800) 882-0052.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund.
Performance Additional Market Index [Text]	rr_PerformanceAdditionalMarketIndex	The table compares the Fund’s performance to that of the S&P® Municipal Bond Index and the S&P® Limited Maturity Municipal Bond Index, which are relevant to the Fund because they have characteristics similar to the Fund’s investment strategies.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	(800) 882-0052
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.blackrock.com/funds
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	As with all such investments, past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading]	rr_BarChartHeading	Investor A1 Shares ANNUAL TOTAL RETURNS BlackRock Short-Term Municipal Fund As of 12/31
Bar Chart Does Not Reflect Sales Loads [Text]	rr_BarChartDoesNotReflectSalesLoads	Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown.
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	During the ten-year period shown in the bar chart, the highest return for a quarter was 1.65% (quarter ended December 31, 2008) and the lowest return for a quarter was -0.45% (quarter ended June 30, 2013). The year-to-date return as of September 30, 2014 was 0.26%.
Performance Table Heading	rr_PerformanceTableHeading	As of 12/31/13 Average Annual Total Returns
Performance Table Does Reflect Sales Loads	rr_PerformanceTableDoesReflectSalesLoads	However, the table includes all applicable fees and sales charges.
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Investor A1 | BlackRock Short-Term Municipal Fund | Investor A1 Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	1.00%
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none
Management Fee	rr_ManagementFeesOverAssets	0.33%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.10%
Other Expenses	rr_OtherExpensesOverAssets	0.11%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.54%
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.02%)	[1]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	0.52%	[1]
1 Year	rr_ExpenseExampleYear01	$ 153
3 Years	rr_ExpenseExampleYear03	269
5 Years	rr_ExpenseExampleYear05	397
10 Years	rr_ExpenseExampleYear10	$ 768
2004	rr_AnnualReturn2004	0.62%
2005	rr_AnnualReturn2005	1.20%
2006	rr_AnnualReturn2006	2.85%
2007	rr_AnnualReturn2007	3.87%
2008	rr_AnnualReturn2008	3.70%
2009	rr_AnnualReturn2009	3.60%
2010	rr_AnnualReturn2010	0.53%
2011	rr_AnnualReturn2011	1.53%
2012	rr_AnnualReturn2012	0.57%
2013	rr_AnnualReturn2013	0.46%
Year to Date Return, Label	rr_YearToDateReturnLabel	year-to-date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2014
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.26%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2008
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.65%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Jun. 30, 2013
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(0.45%)
1 Year	rr_AverageAnnualReturnYear01	(0.54%)
5 Years	rr_AverageAnnualReturnYear05	1.13%
10 Years	rr_AverageAnnualReturnYear10	1.78%
Investor A1 | BlackRock Short-Term Municipal Fund | Return After Taxes on Distributions | Investor A1 Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(0.54%)
5 Years	rr_AverageAnnualReturnYear05	1.13%
10 Years	rr_AverageAnnualReturnYear10	1.78%
Investor A1 | BlackRock Short-Term Municipal Fund | Return After Taxes on Distributions and Sale of Fund Shares | Investor A1 Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(0.11%)
5 Years	rr_AverageAnnualReturnYear05	1.12%
10 Years	rr_AverageAnnualReturnYear10	1.79%
Investor A1 | BlackRock Short-Term Municipal Fund | S&P ® Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(2.55%)
5 Years	rr_AverageAnnualReturnYear05	6.34%
10 Years	rr_AverageAnnualReturnYear10	4.33%
Investor A1 | BlackRock Short-Term Municipal Fund | S&P ® Limited Maturity Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	1.02%
5 Years	rr_AverageAnnualReturnYear05	2.18%
10 Years	rr_AverageAnnualReturnYear10	2.68%

[1] As described in the "Management of the Funds" section of the Fund's prospectus on page 31, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 0.52% (for Investor A1 Shares) of average daily net assets until November 1, 2015. The contractual agreement may be terminated upon 90 days' notice by a majority of the non-interested trustees of the Fund or by a vote of a majority of the outstanding voting securities of the Fund.

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Investor A1 | BlackRock Short-Term Municipal Fund

Fund Overview

Key Facts About BlackRock Short-Term Municipal Fund

Investment Objective

The investment objective of BlackRock Short-Term Municipal Fund (the “Short-Term Fund” or the “Fund”) is to provide shareholders with as high a level of income exempt from Federal income taxes as is consistent with the investment policies of the Fund.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold Investor A1 Shares of the Fund.

Shareholder Fees
(fees paid directly from your investment)

Shareholder Fees	Investor A1 BlackRock Short-Term Municipal Fund Investor A1 Shares
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	1.00%
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	none

Annual Fund Operating Expenses
(expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses		Investor A1 BlackRock Short-Term Municipal Fund Investor A1 Shares
Management Fee		0.33%
Distribution and/or Service (12b-1) Fees		0.10%
Other Expenses		0.11%
Total Annual Fund Operating Expenses		0.54%
Fee Waivers and/or Expense Reimbursements	[1]	(0.02%)
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	[1]	0.52%

Example:

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example (USD $)	1 Year	3 Years	5 Years	10 Years
Investor A1 BlackRock Short-Term Municipal Fund Investor A1 Shares	153	269	397	768

Portfolio Turnover:

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 56% of the average value of its portfolio.

Principal Investment Strategies of the Fund

Under normal circumstances, the Short-Term Fund seeks to achieve its objective by investing at least 80% of its assets in municipal bonds. Municipal bonds include debt obligations issued by or on behalf of a governmental entity or other qualifying issuer that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax). Municipal bonds may be obligations of a variety of issuers, including governmental entities or other qualifying issuers. Issuers may be states, territories and possessions of the United States and the District of Columbia and their political subdivisions, agencies and instrumentalities. Municipal bonds also include short-term tax-exempt obligations like municipal notes and variable rate demand obligations.

The Short-Term Fund invests primarily in investment grade municipal bonds or municipal notes, including variable rate demand obligations. Investment grade bonds are securities which are rated in the four highest categories by at least one of the major rating agencies or determined by the management team to be of similar quality. Generally, the higher the rating of a bond, the higher the likelihood that interest and principal payments will be made on time. Municipal notes are shorter-term municipal debt obligations that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax) and that have a maturity that is generally one year or less.

The Fund will maintain a dollar weighted maturity of no more than two years. Because of their shorter maturities, the Fund's investments will not usually be as sensitive to changes in prevailing interest rates as are long-term municipal bonds. Fluctuations in interest rates on short-term municipal bonds may, however, vary more widely from time to time than those on long-term municipal bonds.

Principal Risks of Investing in the Fund

Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund.

• Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things.

  Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund's investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund's investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund's net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the "full faith and credit" of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund's performance.
  
  Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make principal and interest payments when due. Changes in an issuer's credit rating or the market's perception of an issuer's creditworthiness may also affect the value of the Fund's investment in that issuer. The degree of credit risk depends on the issuer's financial condition and on the terms of the securities.
  
  Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall.
  
  Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields.

• Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund's portfolio will be magnified when the Fund uses leverage.

• Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund's investment in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund's principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquid investments may become illiquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions.

• Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money.

• Municipal Securities Risks — Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include:

  General Obligation Bonds Risks — Timely payments depend on the issuer's credit quality, ability to raise tax revenues and ability to maintain an adequate tax base.
  
  Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source.
  
  Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its faith, credit and taxing power for repayment.
  
  Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality.
  
  Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money.
  
  Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property.
  
  Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers' bond counsel and, in the case of derivative securities, sponsors' counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities.

• Variable Rate Demand Obligations Risk — Variable rate demand obligations are floating rate securities that combine an interest in a long-term municipal bond with a right to demand payment before maturity from a bank or other financial institution. If the bank or financial institution is unable to pay, the Fund may lose money.

Performance Information

The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the S&P^® Municipal Bond Index and the S&P^® Limited Maturity Municipal Bond Index, which are relevant to the Fund because they have characteristics similar to the Fund’s investment strategies. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com/funds or can be obtained by phone at (800) 882-0052.

Investor A1 Shares
ANNUAL TOTAL RETURNS
BlackRock Short-Term Municipal Fund
As of 12/31

During the ten-year period shown in the bar chart, the highest return for a quarter was 1.65% (quarter ended December 31, 2008) and the lowest return for a quarter was -0.45% (quarter ended June 30, 2013). The year-to-date return as of September 30, 2014 was 0.26%.

As of 12/31/13
Average Annual Total Returns

Average Annual Total Returns Investor A1 BlackRock Short-Term Municipal Fund	1 Year	5 Years	10 Years
Investor A1 Shares	(0.54%)	1.13%	1.78%
Investor A1 Shares Return After Taxes on Distributions	(0.54%)	1.13%	1.78%
Investor A1 Shares Return After Taxes on Distributions and Sale of Fund Shares	(0.11%)	1.12%	1.79%
S&P ® Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)	(2.55%)	6.34%	4.33%
S&P ® Limited Maturity Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)	1.02%	2.18%	2.68%

After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.

[1] As described in the "Management of the Funds" section of the Fund's prospectus on page 31, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 0.52% (for Investor A1 Shares) of average daily net assets until November 1, 2015. The contractual agreement may be terminated upon 90 days' notice by a majority of the non-interested trustees of the Fund or by a vote of a majority of the outstanding voting securities of the Fund.

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Service | BlackRock National Municipal Fund

Fund Overview

Key Facts About BlackRock National Municipal Fund

Investment Objective

The investment objective of BlackRock National Municipal Fund (the “Fund”) is to provide shareholders with as high a level of income exempt from Federal income taxes as is consistent with the investment policies of the Fund.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold Service Shares of the Fund.

Shareholder Fees
(fees paid directly from your investment)

Shareholder Fees	Service BlackRock National Municipal Fund Service Shares
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	none

Annual Fund Operating Expenses
(expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses	Service BlackRock National Municipal Fund Service Shares
Management Fee	0.48%
Distribution and/or Service (12b-1) Fees	0.25%
Other Expenses	0.17%
Interest Expense	0.06%
Miscellaneous Other Expenses	0.11%
Total Annual Fund Operating Expenses	0.90%

Example:

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example (USD $)	1 Year	3 Years	5 Years	10 Years
Service BlackRock National Municipal Fund Service Shares	92	287	498	1,108

Portfolio Turnover:

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 35% of the average value of its portfolio.

Principal Investment Strategies of the Fund

Under normal circumstances, the Fund seeks to achieve its objective by investing at least 80% of its assets in municipal bonds. Municipal bonds include debt obligations issued by or on behalf of a governmental entity or other qualifying issuer that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax). Municipal bonds may be obligations of a variety of issuers, including governmental entities or other qualifying issuers. Issuers may be states, territories and possessions of the United States and the District of Columbia and their political subdivisions, agencies and instrumentalities. Municipal bonds also include short-term tax-exempt obligations like municipal notes and variable rate demand obligations.

The Fund may invest in municipal bonds rated in any rating category or in unrated municipal bonds. Although Fund management presently intends to invest at least 65% of the Fund's net assets in municipal bonds rated investment grade or in unrated municipal bonds that Fund management believes are of comparable quality, it is possible that in the future the Fund could invest up to 100% of its assets in "junk bonds." Investment grade bonds are securities which are rated in the four highest categories by at least one of the major rating agencies or determined by the management team to be of similar quality. The Fund does not intend to invest more than 35% of its net assets in junk bonds or in unrated bonds that Fund management believes are of comparable quality. This is a non-fundamental policy and may be changed by the Board at any time. The Fund will usually invest in municipal bonds that have a maturity of five years or longer.

The Fund may engage in transactions in certain derivatives, such as financial futures contracts and options thereon, indexed and inverse floating rate obligations and swap agreements, including credit default swap agreements. The Fund may use derivative instruments to hedge its investments or to seek to enhance returns. The Fund may leverage its assets through the use of proceeds received through tender option bond transactions. In a tender option bond transaction, a third party sponsor establishes a special purpose entity, the tender option bond trust (a "TOB Trust"), into which the Fund transfers municipal bonds or other municipal securities. The Fund may invest in tender option bonds and residual interest tender option bonds and may also invest in securities the return of which is inversely related to changes in an interest rate ("inverse floaters"). The Fund will look through to the underlying municipal bond held by a TOB Trust for purposes of the Fund's 80% policy. The Fund may invest in tender option bonds on either a non-recourse or recourse basis. If the Fund invests in a TOB Trust on a recourse basis it will bear the risk of loss with respect to any liquidation of the TOB Trust.

Principal Risks of Investing in the Fund

Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund.

• Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things.

  Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund's investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund's investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund's net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the "full faith and credit" of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund's performance.
  
  Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make principal and interest payments when due. Changes in an issuer's credit rating or the market's perception of an issuer's creditworthiness may also affect the value of the Fund's investment in that issuer. The degree of credit risk depends on the issuer's financial condition and on the terms of the securities.
  
  Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall.
  
  Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields.

• Derivatives Risk — The Fund's use of derivatives may reduce the Fund's returns and/or increase volatility. Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Fund's use of derivatives is that the fluctuations in their values may not correlate perfectly with the overall securities markets. Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will not fulfill its contractual obligation. The possible lack of a liquid secondary market for derivatives and the resulting inability of the Fund to sell or otherwise close a derivatives position could expose the Fund to losses and could make derivatives more difficult for the Fund to value accurately. Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them. Derivatives also may expose the Fund to greater risk and increase its costs. Certain transactions in derivatives involve substantial leverage risk and may expose the Fund to potential losses that exceed the amount originally invested by the Fund. The U.S. Government is in the process of adopting and implementing regulations governing derivatives markets, including mandatory clearing of certain derivatives, margin, reporting and registration requirements. The ultimate impact of the regulations remains unclear. Additional U.S. or other regulations may make derivatives more costly, may limit the availability of derivatives, or may otherwise adversely affect the value or performance of derivatives. Certain aspects of the tax treatment of derivative instruments, including swap agreements and commodity-linked derivative instruments are currently unclear and may be affected by changes in legislation, regulations or other legally binding authority that could affect the character, timing and amount of the Fund's taxable income or gains and distributions.

• Junk Bonds Risk — Although junk bonds generally pay higher rates of interest than investment grade bonds, junk bonds are high risk investments that may cause income and principal losses for the Fund.

• Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund's portfolio will be magnified when the Fund uses leverage.

• Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund's investment in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund's principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquid investments may become illiquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions.

• Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money.

• Municipal Securities Risks — Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include:

  General Obligation Bonds Risks — Timely payments depend on the issuer's credit quality, ability to raise tax revenues and ability to maintain an adequate tax base.
  
  Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source.
  
  Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its faith, credit and taxing power for repayment.
  
  Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality.
  
  Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money.
  
  Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property.
  
  Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers' bond counsel and, in the case of derivative securities, sponsors' counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities.

• Tender Option Bonds and Related Securities Risk — The Fund's use of tender option bonds may reduce the Fund's return and/or increase volatility. Investments in tender option bonds expose the Fund to counterparty risk and leverage risk. An investment in tender option bonds typically will involve greater risk than an investment in a municipal fixed rate security, including the risk of loss of principal. Distributions on residual interest tender option bonds ("TOB Residuals") and inverse floaters will bear an inverse relationship to short-term municipal security interest rates. Distributions on TOB Residuals and inverse floaters paid to the Fund will be reduced or, in the extreme, eliminated as short-term municipal interest rates rise and will increase when short-term municipal interest rates fall. TOB Residuals and inverse floaters generally will underperform the market for fixed rate municipal securities in a rising interest rate environment.

Performance Information

The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the S&P^® Municipal Bond Index and the Custom National Index. Effective as of September 1, 2014, a customized benchmark, referred to as the “Custom National Index,” was added as a performance benchmark against which the Fund measures its performance. Fund management believes that this performance benchmark more accurately reflects the investment strategy of the Fund. The returns for Service Shares prior to July 18, 2011, the commencement of operations of Service Shares, are based upon performance of the Fund’s Institutional Shares, as adjusted to reflect the distribution and service (12b-1) fees applicable to Service Shares. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com/funds or can be obtained by phone at (800) 882-0052.

Service Shares
ANNUAL TOTAL RETURNS
BlackRock National Municipal Fund
As of 12/31

During the ten-year period shown in the bar chart, the highest return for a quarter was 7.53% (quarter ended September 30, 2009) and the lowest return for a quarter was -4.70% (quarter ended December 31, 2010). The year-to-date return as of September 30, 2014 was 8.95%.

As of 12/31/13
Average Annual Total Returns

Average Annual Total Returns Service BlackRock National Municipal Fund		1 Year	5 Years	10 Years
Service Shares		(3.36%)	7.28%	4.26%
Service Shares Return After Taxes on Distributions		(3.36%)	7.28%	4.26%
Service Shares Return After Taxes on Distributions and Sale of Fund Shares		(0.41%)	6.73%	4.33%
S&P ® Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)		(2.55%)	6.34%	4.33%
Custom National Index (Reflects no deduction for fees, expenses or taxes)	[1]	(3.61%)	6.11%	4.21%

After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.

[1] The Custom National Index reflects the returns of the S&P® Municipal Bond Index for periods prior to January 1, 2013, and the returns of only those bonds in the S&P® Municipal Bond Index that have maturities greater than 5 years for periods subsequent to January 1, 2013.

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BlackRock | BlackRock Short-Term Municipal Fund

Fund Overview

Key Facts About BlackRock Short-Term Municipal Fund

Investment Objective

The investment objective of BlackRock Short-Term Municipal Fund (the “Short-Term Fund” or the “Fund”) is to provide shareholders with as high a level of income exempt from Federal income taxes as is consistent with the investment policies of the Fund.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold BlackRock Shares of the Fund.

Shareholder Fees
(fees paid directly from your investment)

Shareholder Fees	BlackRock BlackRock Short-Term Municipal Fund BlackRock Shares
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	none

Annual Fund Operating Expenses
(expenses that you pay each year as a
percentage of the value of your investment)

Annual Fund Operating Expenses	BlackRock BlackRock Short-Term Municipal Fund BlackRock Shares
Management Fee	0.33%
Distribution and/or Service (12b-1) Fees	none
Other Expenses	0.06%
Total Annual Fund Operating Expenses	0.39%

Example:

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example (USD $)	1 Year	3 Years	5 Years	10 Years
BlackRock BlackRock Short-Term Municipal Fund BlackRock Shares	40	125	219	493

Portfolio Turnover:

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 56% of the average value of its portfolio.

Principal Investment Strategies of the Fund

Under normal circumstances, the Short-Term Fund seeks to achieve its objective by investing at least 80% of its assets in municipal bonds. Municipal bonds include debt obligations issued by or on behalf of a governmental entity or other qualifying issuer that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax). Municipal bonds may be obligations of a variety of issuers, including governmental entities or other qualifying issuers. Issuers may be states, territories and possessions of the United States and the District of Columbia and their political subdivisions, agencies and instrumentalities. Municipal bonds also include short-term tax-exempt obligations like municipal notes and variable rate demand obligations.

The Short-Term Fund invests primarily in investment grade municipal bonds or municipal notes, including variable rate demand obligations. Investment grade bonds are securities which are rated in the four highest categories by at least one of the major rating agencies or determined by the management team to be of similar quality. Generally, the higher the rating of a bond, the higher the likelihood that interest and principal payments will be made on time. Municipal notes are shorter-term municipal debt obligations that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax) and that have a maturity that is generally one year or less.

The Fund will maintain a dollar weighted maturity of no more than two years. Because of their shorter maturities, the Fund's investments will not usually be as sensitive to changes in prevailing interest rates as are long-term municipal bonds. Fluctuations in interest rates on short-term municipal bonds may, however, vary more widely from time to time than those on long-term municipal bonds.

Principal Risks of Investing in the Fund

Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund.

• Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things.

  Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund's investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund's investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund's net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the "full faith and credit" of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund's performance.
  
  Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make principal and interest payments when due. Changes in an issuer's credit rating or the market's perception of an issuer's creditworthiness may also affect the value of the Fund's investment in that issuer. The degree of credit risk depends on the issuer's financial condition and on the terms of the securities.
  
  Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall.
  
  Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields.

• Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund's portfolio will be magnified when the Fund uses leverage.

• Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund's investment in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund's principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquid investments may become illiquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions.

• Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money.

• Municipal Securities Risks — Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include:

  General Obligation Bonds Risks — Timely payments depend on the issuer's credit quality, ability to raise tax revenues and ability to maintain an adequate tax base.
  
  Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source.
  
  Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its faith, credit and taxing power for repayment.
  
  Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality.
  
  Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money.
  
  Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property.
  
  Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers' bond counsel and, in the case of derivative securities, sponsors' counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities.

• Variable Rate Demand Obligations Risk — Variable rate demand obligations are floating rate securities that combine an interest in a long-term municipal bond with a right to demand payment before maturity from a bank or other financial institution. If the bank or financial institution is unable to pay, the Fund may lose money.

Performance Information

The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the S&P^® Municipal Bond Index and the S&P^® Limited Maturity Municipal Bond Index, which are relevant to the Fund because they have characteristics similar to the Fund’s investment strategies. The returns for BlackRock Shares prior to October 2, 2006, the commencement of operations of BlackRock Shares, are based upon performance of the Fund’s Institutional Shares, as adjusted to reflect class-specific fees applicable to BlackRock Shares. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com/funds or can be obtained by phone at (800) 882-0052.

BlackRock Shares
ANNUAL TOTAL RETURNS
BlackRock Short-Term Municipal Fund
As of 12/31

During the ten-year period shown in the bar chart, the highest return for a quarter was 1.65% (quarter ended December 31, 2008) and the lowest return for a quarter was -0.43% (quarter ended June 30, 2013). The year-to-date return as of September 30, 2014 was 0.35%.

As of 12/31/13
Average Annual Total Returns

Average Annual Total Returns BlackRock BlackRock Short-Term Municipal Fund	1 Year	5 Years	10 Years
BlackRock Shares	0.48%	1.42%	1.98%
BlackRock Shares Return After Taxes on Distributions	0.48%	1.42%	1.98%
BlackRock Shares Return After Taxes on Distributions and Sale of Fund Shares	0.53%	1.38%	1.97%
S&P ® Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)	(2.55%)	6.34%	4.33%
S&P ® Limited Maturity Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)	1.02%	2.18%	2.68%

After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	BLACKROCK MUNICIPAL BOND FUND, INC.
Prospectus Date	rr_ProspectusDate	Oct. 28, 2014
Investor A, C and Institutional | BlackRock Short-Term Municipal Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Overview Key Facts About BlackRock Short-Term Municipal Fund
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The investment objective of BlackRock Short-Term Municipal Fund (the “Short-Term Fund” or the “Fund”) is to provide shareholders with as high a level of income exempt from Federal income taxes as is consistent with the investment policies of the Fund.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the fund complex advised by BlackRock Advisors, LLC (“BlackRock”) or its affiliates. More information about these and other discounts is available from your financial professional or your selected securities dealer, broker, investment adviser, service provider or industry professional (including BlackRock, The PNC Financial Services Group, Inc. (“PNC”) and their respective affiliates) (each a “Financial Intermediary”) and in the “Details About the Share Classes” section on page 33 of the Fund’s prospectus and in the “Purchase of Shares” section on page II-67 of Part II of the Fund’s Statement of Additional Information.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	November 1, 2015
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover:
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 56% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	56.00%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	A contingent deferred sales charge ("CDSC") of 0.50% is assessed on certain redemptions of Investor A Shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase of an investment of $250,000 or more.
Expense Breakpoint Discounts [Text]	rr_ExpenseBreakpointDiscounts	You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the fund complex advised by BlackRock Advisors, LLC (“BlackRock”) or its affiliates.
Expense Breakpoint, Minimum Investment Required [Amount]	rr_ExpenseBreakpointMinimumInvestmentRequiredAmount	$ 50,000
Expense Example [Heading]	rr_ExpenseExampleHeading	Example:
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example, No Redemption, By Year, Caption [Text]	rr_ExpenseExampleNoRedemptionByYearCaption	You would pay the following expenses if you did not redeem your shares:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies of the Fund
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the Short-Term Fund seeks to achieve its objective by investing at least 80% of its assets in municipal bonds. Municipal bonds include debt obligations issued by or on behalf of a governmental entity or other qualifying issuer that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax). Municipal bonds may be obligations of a variety of issuers, including governmental entities or other qualifying issuers. Issuers may be states, territories and possessions of the United States and the District of Columbia and their political subdivisions, agencies and instrumentalities. Municipal bonds also include short-term tax-exempt obligations like municipal notes and variable rate demand obligations. The Short-Term Fund invests primarily in investment grade municipal bonds or municipal notes, including variable rate demand obligations. Investment grade bonds are securities which are rated in the four highest categories by at least one of the major rating agencies or determined by the management team to be of similar quality. Generally, the higher the rating of a bond, the higher the likelihood that interest and principal payments will be made on time. Municipal notes are shorter-term municipal debt obligations that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax) and that have a maturity that is generally one year or less. The Fund will maintain a dollar weighted maturity of no more than two years. Because of their shorter maturities, the Fund's investments will not usually be as sensitive to changes in prevailing interest rates as are long-term municipal bonds. Fluctuations in interest rates on short-term municipal bonds may, however, vary more widely from time to time than those on long-term municipal bonds.
Risk [Heading]	rr_RiskHeading	Principal Risks of Investing in the Fund
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund. Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things. Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund's investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund's investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund's net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the "full faith and credit" of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund's performance. Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make principal and interest payments when due. Changes in an issuer's credit rating or the market's perception of an issuer's creditworthiness may also affect the value of the Fund's investment in that issuer. The degree of credit risk depends on the issuer's financial condition and on the terms of the securities. Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall. Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields. Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund's portfolio will be magnified when the Fund uses leverage. Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund's investment in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund's principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquid investments may become illiquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions. Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money. Municipal Securities Risks — Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include: General Obligation Bonds Risks — Timely payments depend on the issuer's credit quality, ability to raise tax revenues and ability to maintain an adequate tax base. Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source. Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its faith, credit and taxing power for repayment. Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality. Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money. Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property. Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers' bond counsel and, in the case of derivative securities, sponsors' counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities. Variable Rate Demand Obligations Risk — Variable rate demand obligations are floating rate securities that combine an interest in a long-term municipal bond with a right to demand payment before maturity from a bank or other financial institution. If the bank or financial institution is unable to pay, the Fund may lose money.
Risk Lose Money [Text]	rr_RiskLoseMoney	You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the S&P® Municipal Bond Index and the S&P® Limited Maturity Municipal Bond Index, which are relevant to the Fund because they have characteristics similar to the Fund’s investment strategies. The returns for Investor A and Investor C Shares prior to October 2, 2006, the commencement of operations of Investor A and Investor C Shares, are based upon the performance of the Institutional Shares, as adjusted to reflect the distribution and service (12b-1) fees applicable to Investor A and Investor C Shares. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com/funds or can be obtained by phone at (800) 882-0052.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	(800) 882-0052
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.blackrock.com/funds
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	As with all such investments, past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading]	rr_BarChartHeading	Institutional Shares ANNUAL TOTAL RETURNS BlackRock Short-Term Municipal Fund As of 12/31
Bar Chart Does Not Reflect Sales Loads [Text]	rr_BarChartDoesNotReflectSalesLoads	Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown.
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	During the ten-year period shown in the bar chart, the highest return for a quarter was 1.67% (quarter ended December 31, 2008) and the lowest return for a quarter was -0.37% (quarter ended June 30, 2004). The year-to-date return as of September 30, 2014 was 0.35%.
Performance Table Heading	rr_PerformanceTableHeading	As of 12/31/13 Average Annual Total Returns
Performance Table Does Reflect Sales Loads	rr_PerformanceTableDoesReflectSalesLoads	However, the table includes all applicable fees and sales charges.
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Performance Table One Class of after Tax Shown [Text]	rr_PerformanceTableOneClassOfAfterTaxShown	After-tax returns are shown for Institutional Shares only, and the after-tax returns for Investor A and Investor C Shares will vary.
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. After-tax returns are shown for Institutional Shares only, and the after-tax returns for Investor A and Investor C Shares will vary.
Investor A, C and Institutional | BlackRock Short-Term Municipal Fund | Investor A Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	3.00%
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none	[1]
Management Fee	rr_ManagementFeesOverAssets	0.33%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other Expenses	rr_OtherExpensesOverAssets	0.09%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.67%
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	none	[2]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	0.67%	[2]
1 Year	rr_ExpenseExampleYear01	366
3 Years	rr_ExpenseExampleYear03	508
5 Years	rr_ExpenseExampleYear05	662
10 Years	rr_ExpenseExampleYear10	1,109
1 Year	rr_AverageAnnualReturnYear01	(2.80%)
5 Years	rr_AverageAnnualReturnYear05	0.56%
10 Years	rr_AverageAnnualReturnYear10	1.42%
Investor A, C and Institutional | BlackRock Short-Term Municipal Fund | Investor C Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOfferingPrice	1.00%	[3]
Management Fee	rr_ManagementFeesOverAssets	0.33%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	1.00%
Other Expenses	rr_OtherExpensesOverAssets	0.12%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.45%
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	none	[2]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	1.45%	[2]
1 Year	rr_ExpenseExampleYear01	248
3 Years	rr_ExpenseExampleYear03	459
5 Years	rr_ExpenseExampleYear05	792
10 Years	rr_ExpenseExampleYear10	1,735
1 Year	rr_ExpenseExampleNoRedemptionYear01	148
3 Years	rr_ExpenseExampleNoRedemptionYear03	459
5 Years	rr_ExpenseExampleNoRedemptionYear05	792
10 Years	rr_ExpenseExampleNoRedemptionYear10	1,735
1 Year	rr_AverageAnnualReturnYear01	(1.59%)
5 Years	rr_AverageAnnualReturnYear05	0.39%
10 Years	rr_AverageAnnualReturnYear10	0.95%
Investor A, C and Institutional | BlackRock Short-Term Municipal Fund | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none
Management Fee	rr_ManagementFeesOverAssets	0.33%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.18%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.51%
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.11%)	[2]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	0.40%	[2]
1 Year	rr_ExpenseExampleYear01	41
3 Years	rr_ExpenseExampleYear03	152
5 Years	rr_ExpenseExampleYear05	274
10 Years	rr_ExpenseExampleYear10	$ 630
2004	rr_AnnualReturn2004	0.72%
2005	rr_AnnualReturn2005	1.40%
2006	rr_AnnualReturn2006	2.85%
2007	rr_AnnualReturn2007	3.97%
2008	rr_AnnualReturn2008	3.80%
2009	rr_AnnualReturn2009	3.72%
2010	rr_AnnualReturn2010	0.65%
2011	rr_AnnualReturn2011	1.75%
2012	rr_AnnualReturn2012	0.58%
2013	rr_AnnualReturn2013	0.58%
Year to Date Return, Label	rr_YearToDateReturnLabel	year-to-date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2014
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.35%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2008
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.67%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Jun. 30, 2004
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(0.37%)
1 Year	rr_AverageAnnualReturnYear01	0.58%
5 Years	rr_AverageAnnualReturnYear05	1.45%
10 Years	rr_AverageAnnualReturnYear10	1.99%
Investor A, C and Institutional | BlackRock Short-Term Municipal Fund | Return After Taxes on Distributions | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	0.58%
5 Years	rr_AverageAnnualReturnYear05	1.45%
10 Years	rr_AverageAnnualReturnYear10	1.99%
Investor A, C and Institutional | BlackRock Short-Term Municipal Fund | Return After Taxes on Distributions and Sale of Fund Shares | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	0.58%
5 Years	rr_AverageAnnualReturnYear05	1.39%
10 Years	rr_AverageAnnualReturnYear10	1.98%
Investor A, C and Institutional | BlackRock Short-Term Municipal Fund | S&P ® Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(2.55%)
5 Years	rr_AverageAnnualReturnYear05	6.34%
10 Years	rr_AverageAnnualReturnYear10	4.33%
Investor A, C and Institutional | BlackRock Short-Term Municipal Fund | S&P ® Limited Maturity Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	1.02%
5 Years	rr_AverageAnnualReturnYear05	2.18%
10 Years	rr_AverageAnnualReturnYear10	2.68%

[1] A contingent deferred sales charge ("CDSC") of 0.50% is assessed on certain redemptions of Investor A Shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase of an investment of $250,000 or more.

[2] As described in the "Management of the Funds" section of the Fund's prospectus on page 48, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 0.69% (for Investor A Shares), 1.55% (for Investor C Shares) and 0.40% (for Institutional Shares) of average daily net assets until November 1, 2015. The contractual agreement may be terminated upon 90 days' notice by a majority of the non-interested trustees of the Fund or by a vote of a majority of the outstanding securities of the Fund.

[3] There is no CDSC on Investor C Shares after one year.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	BLACKROCK MUNICIPAL BOND FUND, INC.
Prospectus Date	rr_ProspectusDate	Oct. 28, 2014
Service | BlackRock National Municipal Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Overview Key Facts About BlackRock National Municipal Fund
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The investment objective of BlackRock National Municipal Fund (the “Fund”) is to provide shareholders with as high a level of income exempt from Federal income taxes as is consistent with the investment policies of the Fund.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Service Shares of the Fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover:
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 35% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	35.00%
Expense Example [Heading]	rr_ExpenseExampleHeading	Example:
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies of the Fund
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the Fund seeks to achieve its objective by investing at least 80% of its assets in municipal bonds. Municipal bonds include debt obligations issued by or on behalf of a governmental entity or other qualifying issuer that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax). Municipal bonds may be obligations of a variety of issuers, including governmental entities or other qualifying issuers. Issuers may be states, territories and possessions of the United States and the District of Columbia and their political subdivisions, agencies and instrumentalities. Municipal bonds also include short-term tax-exempt obligations like municipal notes and variable rate demand obligations. The Fund may invest in municipal bonds rated in any rating category or in unrated municipal bonds. Although Fund management presently intends to invest at least 65% of the Fund's net assets in municipal bonds rated investment grade or in unrated municipal bonds that Fund management believes are of comparable quality, it is possible that in the future the Fund could invest up to 100% of its assets in "junk bonds." Investment grade bonds are securities which are rated in the four highest categories by at least one of the major rating agencies or determined by the management team to be of similar quality. The Fund does not intend to invest more than 35% of its net assets in junk bonds or in unrated bonds that Fund management believes are of comparable quality. This is a non-fundamental policy and may be changed by the Board at any time. The Fund will usually invest in municipal bonds that have a maturity of five years or longer. The Fund may engage in transactions in certain derivatives, such as financial futures contracts and options thereon, indexed and inverse floating rate obligations and swap agreements, including credit default swap agreements. The Fund may use derivative instruments to hedge its investments or to seek to enhance returns. The Fund may leverage its assets through the use of proceeds received through tender option bond transactions. In a tender option bond transaction, a third party sponsor establishes a special purpose entity, the tender option bond trust (a "TOB Trust"), into which the Fund transfers municipal bonds or other municipal securities. The Fund may invest in tender option bonds and residual interest tender option bonds and may also invest in securities the return of which is inversely related to changes in an interest rate ("inverse floaters"). The Fund will look through to the underlying municipal bond held by a TOB Trust for purposes of the Fund's 80% policy. The Fund may invest in tender option bonds on either a non-recourse or recourse basis. If the Fund invests in a TOB Trust on a recourse basis it will bear the risk of loss with respect to any liquidation of the TOB Trust.
Risk [Heading]	rr_RiskHeading	Principal Risks of Investing in the Fund
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund. Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things. Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund's investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund's investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund's net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the "full faith and credit" of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund's performance. Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make principal and interest payments when due. Changes in an issuer's credit rating or the market's perception of an issuer's creditworthiness may also affect the value of the Fund's investment in that issuer. The degree of credit risk depends on the issuer's financial condition and on the terms of the securities. Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall. Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields. Derivatives Risk — The Fund's use of derivatives may reduce the Fund's returns and/or increase volatility. Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Fund's use of derivatives is that the fluctuations in their values may not correlate perfectly with the overall securities markets. Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will not fulfill its contractual obligation. The possible lack of a liquid secondary market for derivatives and the resulting inability of the Fund to sell or otherwise close a derivatives position could expose the Fund to losses and could make derivatives more difficult for the Fund to value accurately. Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them. Derivatives also may expose the Fund to greater risk and increase its costs. Certain transactions in derivatives involve substantial leverage risk and may expose the Fund to potential losses that exceed the amount originally invested by the Fund. The U.S. Government is in the process of adopting and implementing regulations governing derivatives markets, including mandatory clearing of certain derivatives, margin, reporting and registration requirements. The ultimate impact of the regulations remains unclear. Additional U.S. or other regulations may make derivatives more costly, may limit the availability of derivatives, or may otherwise adversely affect the value or performance of derivatives. Certain aspects of the tax treatment of derivative instruments, including swap agreements and commodity-linked derivative instruments are currently unclear and may be affected by changes in legislation, regulations or other legally binding authority that could affect the character, timing and amount of the Fund's taxable income or gains and distributions. Junk Bonds Risk — Although junk bonds generally pay higher rates of interest than investment grade bonds, junk bonds are high risk investments that may cause income and principal losses for the Fund. Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund's portfolio will be magnified when the Fund uses leverage. Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund's investment in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund's principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquid investments may become illiquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions. Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money. Municipal Securities Risks — Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include: General Obligation Bonds Risks — Timely payments depend on the issuer's credit quality, ability to raise tax revenues and ability to maintain an adequate tax base. Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source. Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its faith, credit and taxing power for repayment. Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality. Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money. Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property. Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers' bond counsel and, in the case of derivative securities, sponsors' counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities. Tender Option Bonds and Related Securities Risk — The Fund's use of tender option bonds may reduce the Fund's return and/or increase volatility. Investments in tender option bonds expose the Fund to counterparty risk and leverage risk. An investment in tender option bonds typically will involve greater risk than an investment in a municipal fixed rate security, including the risk of loss of principal. Distributions on residual interest tender option bonds ("TOB Residuals") and inverse floaters will bear an inverse relationship to short-term municipal security interest rates. Distributions on TOB Residuals and inverse floaters paid to the Fund will be reduced or, in the extreme, eliminated as short-term municipal interest rates rise and will increase when short-term municipal interest rates fall. TOB Residuals and inverse floaters generally will underperform the market for fixed rate municipal securities in a rising interest rate environment.
Risk Lose Money [Text]	rr_RiskLoseMoney	You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the S&P® Municipal Bond Index and the Custom National Index. Effective as of September 1, 2014, a customized benchmark, referred to as the “Custom National Index,” was added as a performance benchmark against which the Fund measures its performance. Fund management believes that this performance benchmark more accurately reflects the investment strategy of the Fund. The returns for Service Shares prior to July 18, 2011, the commencement of operations of Service Shares, are based upon performance of the Fund’s Institutional Shares, as adjusted to reflect the distribution and service (12b-1) fees applicable to Service Shares. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com/funds or can be obtained by phone at (800) 882-0052.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The information shows you how the Fund's performance has varied year by year and provides some indication of the risks of investing in the Fund.
Performance Additional Market Index [Text]	rr_PerformanceAdditionalMarketIndex	The table compares the Fund’s performance to that of the S&P® Municipal Bond Index and the Custom National Index. Effective as of September 1, 2014, a customized benchmark, referred to as the “Custom National Index,” was added as a performance benchmark against which the Fund measures its performance. Fund management believes that this performance benchmark more accurately reflects the investment strategy of the Fund.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	(800) 882-0052
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.blackrock.com/funds
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	As with all such investments, past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading]	rr_BarChartHeading	Service Shares ANNUAL TOTAL RETURNS BlackRock National Municipal Fund As of 12/31
Bar Chart Does Not Reflect Sales Loads [Text]	rr_BarChartDoesNotReflectSalesLoads	Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown.
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	During the ten-year period shown in the bar chart, the highest return for a quarter was 7.53% (quarter ended September 30, 2009) and the lowest return for a quarter was -4.70% (quarter ended December 31, 2010). The year-to-date return as of September 30, 2014 was 8.95%.
Performance Table Heading	rr_PerformanceTableHeading	As of 12/31/13 Average Annual Total Returns
Performance Table Does Reflect Sales Loads	rr_PerformanceTableDoesReflectSalesLoads	However, the table includes all applicable fees and sales charges.
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	Actual after-tax returns depend on the investor's tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Service | BlackRock National Municipal Fund | Service Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none
Management Fee	rr_ManagementFeesOverAssets	0.48%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Interest Expense	rr_Component1OtherExpensesOverAssets	0.06%
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.11%
Other Expenses	rr_OtherExpensesOverAssets	0.17%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.90%
1 Year	rr_ExpenseExampleYear01	$ 92
3 Years	rr_ExpenseExampleYear03	287
5 Years	rr_ExpenseExampleYear05	498
10 Years	rr_ExpenseExampleYear10	$ 1,108
2004	rr_AnnualReturn2004	5.23%
2005	rr_AnnualReturn2005	4.54%
2006	rr_AnnualReturn2006	5.12%
2007	rr_AnnualReturn2007	1.32%
2008	rr_AnnualReturn2008	(8.80%)
2009	rr_AnnualReturn2009	17.57%
2010	rr_AnnualReturn2010	2.83%
2011	rr_AnnualReturn2011	11.43%
2012	rr_AnnualReturn2012	9.13%
2013	rr_AnnualReturn2013	(3.36%)
Year to Date Return, Label	rr_YearToDateReturnLabel	year-to-date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2014
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	8.95%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Sep. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	7.53%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2010
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(4.70%)
1 Year	rr_AverageAnnualReturnYear01	(3.36%)
5 Years	rr_AverageAnnualReturnYear05	7.28%
10 Years	rr_AverageAnnualReturnYear10	4.26%
Service | BlackRock National Municipal Fund | Return After Taxes on Distributions | Service Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(3.36%)
5 Years	rr_AverageAnnualReturnYear05	7.28%
10 Years	rr_AverageAnnualReturnYear10	4.26%
Service | BlackRock National Municipal Fund | Return After Taxes on Distributions and Sale of Fund Shares | Service Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(0.41%)
5 Years	rr_AverageAnnualReturnYear05	6.73%
10 Years	rr_AverageAnnualReturnYear10	4.33%
Service | BlackRock National Municipal Fund | S&P ® Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(2.55%)
5 Years	rr_AverageAnnualReturnYear05	6.34%
10 Years	rr_AverageAnnualReturnYear10	4.33%
Service | BlackRock National Municipal Fund | Custom National Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(3.61%)	[1]
5 Years	rr_AverageAnnualReturnYear05	6.11%	[1]
10 Years	rr_AverageAnnualReturnYear10	4.21%	[1]

[1] The Custom National Index reflects the returns of the S&P® Municipal Bond Index for periods prior to January 1, 2013, and the returns of only those bonds in the S&P® Municipal Bond Index that have maturities greater than 5 years for periods subsequent to January 1, 2013.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	BLACKROCK MUNICIPAL BOND FUND, INC.
Prospectus Date	rr_ProspectusDate	Oct. 28, 2014
Investor C1 | BlackRock National Municipal Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Overview Key Facts About BlackRock National Municipal Fund
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The investment objective of BlackRock National Municipal Fund (the “National Fund” or the “Fund”) is to provide shareholders with as high a level of income exempt from Federal income taxes as is consistent with the investment policies of the Fund.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Investor C1 Shares of the Fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	November 1, 2015
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover:
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 35% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	35.00%
Expense Example [Heading]	rr_ExpenseExampleHeading	Example:
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example, No Redemption, By Year, Caption [Text]	rr_ExpenseExampleNoRedemptionByYearCaption	You would pay the following expenses if you did not redeem your shares:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies of the Fund
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the National Fund seeks to achieve its objective by investing at least 80% of its assets in municipal bonds. Municipal bonds include debt obligations issued by or on behalf of a governmental entity or other qualifying issuer that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax). Municipal bonds may be obligations of a variety of issuers, including governmental entities or other qualifying issuers. Issuers may be states, territories and possessions of the United States and the District of Columbia and their political subdivisions, agencies and instrumentalities. Municipal bonds also include short-term tax-exempt obligations like municipal notes and variable rate demand obligations. The Fund may invest in municipal bonds rated in any rating category or in unrated municipal bonds. Although Fund management presently intends to invest at least 65% of the Fund’s net assets in municipal bonds rated investment grade or in unrated municipal bonds that Fund management believes are of comparable quality, it is possible that in the future the Fund could invest up to 100% of its assets in “junk bonds.” Investment grade bonds are securities which are rated in the four highest categories by at least one of the major rating agencies or determined by the management team to be of similar quality. The Fund does not intend to invest more than 35% of its net assets in junk bonds or in unrated bonds that Fund management believes are of comparable quality. This is a non-fundamental policy and may be changed by the Board at any time. The Fund will usually invest in municipal bonds that have a maturity of five years or longer. The Fund may engage in transactions in certain derivatives, such as financial futures contracts and options thereon, indexed and inverse floating rate obligations and swap agreements, including credit default swap agreements. The Fund may use derivative instruments to hedge its investments or to seek to enhance returns. The Fund may leverage its assets through the use of proceeds received through tender option bond transactions. In a tender option bond transaction, a third party sponsor establishes a special purpose entity, the tender option bond trust (a “TOB Trust”), into which the Fund transfers municipal bonds or other municipal securities. The Fund may invest in tender option bonds and residual interest tender option bonds and may also invest in securities the return of which is inversely related to changes in an interest rate (“inverse floaters”). The Fund will look through to the underlying municipal bond held by a TOB Trust for purposes of the Fund’s 80% policy. The Fund may invest in tender option bonds on either a non-recourse or recourse basis. If the Fund invests in a TOB Trust on a recourse basis it will bear the risk of loss with respect to any liquidation of the TOB Trust.
Risk [Heading]	rr_RiskHeading	Principal Risks of Investing in the Fund
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund. Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things. Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund's investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund's investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund's net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the "full faith and credit" of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund's performance. Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make principal and interest payments when due. Changes in an issuer's credit rating or the market's perception of an issuer's creditworthiness may also affect the value of the Fund's investment in that issuer. The degree of credit risk depends on the issuer's financial condition and on the terms of the securities. Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall. Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields. Derivatives Risk — The Fund's use of derivatives may reduce the Fund's returns and/or increase volatility. Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Fund's use of derivatives is that the fluctuations in their values may not correlate perfectly with the overall securities markets. Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will not fulfill its contractual obligation. The possible lack of a liquid secondary market for derivatives and the resulting inability of the Fund to sell or otherwise close a derivatives position could expose the Fund to losses and could make derivatives more difficult for the Fund to value accurately. Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them. Derivatives also may expose the Fund to greater risk and increase its costs. Certain transactions in derivatives involve substantial leverage risk and may expose the Fund to potential losses that exceed the amount originally invested by the Fund. The U.S. Government is in the process of adopting and implementing regulations governing derivatives markets, including mandatory clearing of certain derivatives, margin, reporting and registration requirements. The ultimate impact of the regulations remains unclear. Additional U.S. or other regulations may make derivatives more costly, may limit the availability of derivatives, or may otherwise adversely affect the value or performance of derivatives. Certain aspects of the tax treatment of derivative instruments, including swap agreements and commodity-linked derivative instruments are currently unclear and may be affected by changes in legislation, regulations or other legally binding authority that could affect the character, timing and amount of the Fund's taxable income or gains and distributions. Junk Bonds Risk — Although junk bonds generally pay higher rates of interest than investment grade bonds, junk bonds are high risk investments that may cause income and principal losses for the Fund. Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund's portfolio will be magnified when the Fund uses leverage. Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund's investment in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund's principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquid investments may become illiquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions. Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money. Municipal Securities Risks — Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include: General Obligation Bonds Risks — Timely payments depend on the issuer's credit quality, ability to raise tax revenues and ability to maintain an adequate tax base. Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source. Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its faith, credit and taxing power for repayment. Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality. Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money. Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property. Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers' bond counsel and, in the case of derivative securities, sponsors' counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities. Tender Option Bonds and Related Securities Risk — The Fund's use of tender option bonds may reduce the Fund's return and/or increase volatility. Investments in tender option bonds expose the Fund to counterparty risk and leverage risk. An investment in tender option bonds typically will involve greater risk than an investment in a municipal fixed rate security, including the risk of loss of principal. Distributions on residual interest tender option bonds ("TOB Residuals") and inverse floaters will bear an inverse relationship to short-term municipal security interest rates. Distributions on TOB Residuals and inverse floaters paid to the Fund will be reduced or, in the extreme, eliminated as short-term municipal interest rates rise and will increase when short-term municipal interest rates fall. TOB Residuals and inverse floaters generally will underperform the market for fixed rate municipal securities in a rising interest rate environment.
Risk Lose Money [Text]	rr_RiskLoseMoney	You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the S&P® Municipal Bond Index and the Custom National Index. Effective as of September 1, 2014, a customized benchmark, referred to as the “Custom National Index,” was added as a performance benchmark against which the Fund measures its performance. Fund management believes that this performance benchmark more accurately reflects the investment strategy of the Fund. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com/funds or can be obtained by phone at (800) 882-0052.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund.
Performance Additional Market Index [Text]	rr_PerformanceAdditionalMarketIndex	The table compares the Fund’s performance to that of the S&P® Municipal Bond Index and the Custom National Index. Effective as of September 1, 2014, a customized benchmark, referred to as the “Custom National Index,” was added as a performance benchmark against which the Fund measures its performance. Fund management believes that this performance benchmark more accurately reflects the investment strategy of the Fund.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	(800) 882-0052
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.blackrock.com/funds
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	As with all such investments, past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading]	rr_BarChartHeading	Investor C1 Shares ANNUAL TOTAL RETURNS BlackRock National Municipal Fund As of 12/31
Bar Chart Does Not Reflect Sales Loads [Text]	rr_BarChartDoesNotReflectSalesLoads	Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown.
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	During the ten-year period shown in the bar chart, the highest return for a quarter was 7.39% (quarter ended September 30, 2009) and the lowest return for a quarter was -4.82% (quarter ended December 31, 2010). The year-to-date return as of September 30, 2014 was 8.51%.
Performance Table Heading	rr_PerformanceTableHeading	As of 12/31/13 Average Annual Total Returns
Performance Table Does Reflect Sales Loads	rr_PerformanceTableDoesReflectSalesLoads	However, the table includes all applicable fees and sales charges.
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Investor C1 | BlackRock National Municipal Fund | Investor C1 Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOfferingPrice	1.00%	[1]
Management Fee	rr_ManagementFeesOverAssets	0.48%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.80%
Interest Expense	rr_Component1OtherExpensesOverAssets	0.06%
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.08%
Other Expenses	rr_OtherExpensesOverAssets	0.14%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.42%
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.08%)	[2]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	1.34%	[2]
1 Year	rr_ExpenseExampleYear01	$ 236
3 Years	rr_ExpenseExampleYear03	441
5 Years	rr_ExpenseExampleYear05	769
10 Years	rr_ExpenseExampleYear10	1,695
1 Year	rr_ExpenseExampleNoRedemptionYear01	136
3 Years	rr_ExpenseExampleNoRedemptionYear03	441
5 Years	rr_ExpenseExampleNoRedemptionYear05	769
10 Years	rr_ExpenseExampleNoRedemptionYear10	$ 1,695
2004	rr_AnnualReturn2004	4.64%
2005	rr_AnnualReturn2005	3.96%
2006	rr_AnnualReturn2006	4.53%
2007	rr_AnnualReturn2007	0.77%
2008	rr_AnnualReturn2008	(9.30%)
2009	rr_AnnualReturn2009	16.93%
2010	rr_AnnualReturn2010	2.28%
2011	rr_AnnualReturn2011	10.76%
2012	rr_AnnualReturn2012	9.03%
2013	rr_AnnualReturn2013	(3.65%)
Year to Date Return, Label	rr_YearToDateReturnLabel	year-to-date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2014
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	8.51%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Sep. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	7.39%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2010
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(4.82%)
1 Year	rr_AverageAnnualReturnYear01	(4.58%)
5 Years	rr_AverageAnnualReturnYear05	6.83%
10 Years	rr_AverageAnnualReturnYear10	3.76%
Investor C1 | BlackRock National Municipal Fund | Return After Taxes on Distributions | Investor C1 Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(4.58%)
5 Years	rr_AverageAnnualReturnYear05	6.83%
10 Years	rr_AverageAnnualReturnYear10	3.76%
Investor C1 | BlackRock National Municipal Fund | Return After Taxes on Distributions and Sale of Fund Shares | Investor C1 Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(1.28%)
5 Years	rr_AverageAnnualReturnYear05	6.23%
10 Years	rr_AverageAnnualReturnYear10	3.78%
Investor C1 | BlackRock National Municipal Fund | S&P ® Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(2.55%)
5 Years	rr_AverageAnnualReturnYear05	6.34%
10 Years	rr_AverageAnnualReturnYear10	4.33%
Investor C1 | BlackRock National Municipal Fund | Custom National Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(3.61%)	[3]
5 Years	rr_AverageAnnualReturnYear05	6.11%	[3]
10 Years	rr_AverageAnnualReturnYear10	4.21%	[3]

[1] There is no CDSC on Investor C1 Shares after one year.

[2] As described in the "Management of the Funds" section of the Fund's prospectus on page 31, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 1.28% (for Investor C1 Shares) of average daily net assets until November 1, 2015. The contractual agreement may be terminated upon 90 days' notice by a majority of the non-interested directors of the Fund or by a vote of a majority of the outstanding voting securities of the Fund.

[3] The Custom National Index reflects the returns of the S&P® Municipal Bond Index for periods prior to January 1, 2013, and the returns of only those bonds in the S&P® Municipal Bond Index that have maturities greater than 5 years for periods subsequent to January 1, 2013.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	BLACKROCK MUNICIPAL BOND FUND, INC.
Prospectus Date	rr_ProspectusDate	Oct. 28, 2014
BlackRock High Yield Municipal Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Overview Key Facts About BlackRock High Yield Municipal Fund
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The investment objective of BlackRock High Yield Municipal Fund (the “High Yield Fund” or the “Fund”) is to provide shareholders with as high a level of income exempt from Federal income taxes as is consistent with the investment policies of the Fund.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $100,000 in the fund complex advised by BlackRock Advisors, LLC (“BlackRock”) or its affiliates. More information about these and other discounts is available from your financial professional or your selected securities dealer, broker, investment adviser, service provider or industry professional (including BlackRock, The PNC Financial Services Group, Inc. (“PNC”) and their respective affiliates) (each a “Financial Intermediary”) and in the “Details About the Share Classes” section on page 33 of the Fund’s prospectus and in the “Purchase of Shares” section on page II-67 of Part II of the Fund’s Statement of Additional Information.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover:
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 40% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	40.00%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	A contingent deferred sales charge ("CDSC") of 1.00% is assessed on certain redemptions of Investor A Shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase of an investment of $1,000,000 or more.
Expense Breakpoint Discounts [Text]	rr_ExpenseBreakpointDiscounts	You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $100,000 in the fund complex advised by BlackRock Advisors, LLC (“BlackRock”) or its affiliates.
Expense Breakpoint, Minimum Investment Required [Amount]	rr_ExpenseBreakpointMinimumInvestmentRequiredAmount	$ 100,000
Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text]	rr_ExpensesNotCorrelatedToRatioDueToAcquiredFundFees	The Total Annual Fund Operating Expenses do not correlate to the ratio of expenses to average net assets given in the Fund's most recent annual report, which does not include Acquired Fund Fees and Expenses.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example:
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example, No Redemption, By Year, Caption [Text]	rr_ExpenseExampleNoRedemptionByYearCaption	You would pay the following expenses if you did not redeem your shares:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies of the Fund
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the High Yield Fund seeks to achieve its objective by investing at least 80% of its assets in municipal bonds. Municipal bonds include debt obligations issued by or on behalf of a governmental entity or other qualifying issuer that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax). Municipal bonds may be obligations of a variety of issuers, including governmental entities or other qualifying issuers. Issuers may be states, territories and possessions of the United States and the District of Columbia and their political subdivisions, agencies and instrumentalities. Municipal bonds also include short-term tax-exempt obligations like municipal notes and variable rate demand obligations. The High Yield Fund may invest in municipal bonds rated in any rating category or in unrated municipal bonds. Although the Fund may invest in municipal bonds in any rating category, Fund management presently intends to invest at least 65% of the Fund's net assets in medium- to low-quality bonds as rated by at least one independent rating agency (BBB or lower by Standard & Poor's ("S&P") or Fitch Ratings, Inc. ("Fitch") or Baa or lower by Moody's Investors Service, Inc. ("Moody's")), or if unrated, judged to be of comparable quality by BlackRock. Obligations rated below BBB or Baa are commonly known as "junk bonds." It is possible that the Fund could invest up to 100% of its assets in "junk bonds." The Fund may also invest up to 10% of its assets in municipal bonds that are distressed securities. Distressed securities are securities that are the subject of bankruptcy proceedings or otherwise in default as to the repayment of principal and/or payment of interest at the time of acquisition or are rated in the lowest rating categories by at least one independent rating agency (CC or lower by S&P or Fitch or Ca or lower by Moody's), or if unrated, judged to be of comparable quality by BlackRock. The Fund will usually invest in municipal bonds that have a maturity of five years or longer.
Risk [Heading]	rr_RiskHeading	Principal Risks of Investing in the Fund
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund. Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things. Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund's investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund's investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund's net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the "full faith and credit" of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund's performance. Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make principal and interest payments when due. Changes in an issuer's credit rating or the market's perception of an issuer's creditworthiness may also affect the value of the Fund's investment in that issuer. The degree of credit risk depends on the issuer's financial condition and on the terms of the securities. Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall. Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields. Distressed Securities Risk — Distressed securities are speculative and involve substantial risks in addition to the risks of investing in junk bonds. The Fund will generally not receive interest payments on the distressed securities and may incur costs to protect its investment. In addition, distressed securities involve the substantial risk that principal will not be repaid. These securities may present a substantial risk of default or may be in default at the time of investment. The Fund may incur additional expenses to the extent it is required to seek recovery upon a default in the payment of principal of or interest on its portfolio holdings. In any reorganization or liquidation proceeding relating to a portfolio company, the Fund may lose its entire investment or may be required to accept cash or securities with a value less than its original investment. Distressed securities and any securities received in an exchange for such securities may be subject to restrictions on resale. Junk Bonds Risk — Although junk bonds generally pay higher rates of interest than investment grade bonds, junk bonds are high risk investments that may cause income and principal losses for the Fund. Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund's portfolio will be magnified when the Fund uses leverage. Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund's investment in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund's principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquid investments may become illiquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions. Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money. Municipal Securities Risks — Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include: General Obligation Bonds Risks — Timely payments depend on the issuer's credit quality, ability to raise tax revenues and ability to maintain an adequate tax base. Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source. Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its faith, credit and taxing power for repayment. Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality. Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money. Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property. Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers' bond counsel and, in the case of derivative securities, sponsors' counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities.
Risk Lose Money [Text]	rr_RiskLoseMoney	You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the S&P® Municipal Bond Index, the S&P® Customized High Yield Municipal Bond Index, and the Custom High Yield Index. Effective as of January 1, 2014, a customized benchmark, referred to as the “Custom High Yield Index,” replaced the S&P® Customized High Yield Municipal Bond Index as a performance benchmark against which the Fund measures its performance. Fund management believes that this performance benchmark more accurately reflects the investment strategy of the Fund. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com/funds or can be obtained by phone at (800) 882-0052.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	(800) 882-0052
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.blackrock.com/funds
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	As with all such investments, past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading]	rr_BarChartHeading	Institutional Shares ANNUAL TOTAL RETURNS BlackRock High Yield Municipal Fund As of 12/31
Bar Chart Does Not Reflect Sales Loads [Text]	rr_BarChartDoesNotReflectSalesLoads	Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown.
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	During the period shown in the bar chart, the highest return for a quarter was 16.53% (quarter ended September 30, 2009) and the lowest return for a quarter was -19.35% (quarter ended December 31, 2008). The year-to-date return as of September 30, 2014 was 14.49%.
Performance Table Heading	rr_PerformanceTableHeading	As of 12/31/13 Average Annual Total Returns
Performance Table Does Reflect Sales Loads	rr_PerformanceTableDoesReflectSalesLoads	However, the table includes all applicable fees and sales charges.
Performance Table Market Index Changed	rr_PerformanceTableMarketIndexChanged	The table compares the Fund's performance to that of the S&P® Municipal Bond Index, the S&P® Customized High Yield Municipal Bond Index, and the Custom High Yield Index. Effective as of January 1, 2014, a customized benchmark, referred to as the "Custom High Yield Index," replaced the S&P® Customized High Yield Municipal Bond Index as a performance benchmark against which the Fund measures its performance. Fund management believes that this performance benchmark more accurately reflects the investment strategy of the Fund.
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Performance Table One Class of after Tax Shown [Text]	rr_PerformanceTableOneClassOfAfterTaxShown	After-tax returns are shown for Institutional Shares only, and the after-tax returns for Investor A and Investor C Shares will vary.
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. After-tax returns are shown for Institutional Shares only, and the after-tax returns for Investor A and Investor C Shares will vary.
BlackRock High Yield Municipal Fund | Investor A Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	4.25%
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none	[1]
Management Fee	rr_ManagementFeesOverAssets	0.54%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Interest Expense	rr_Component1OtherExpensesOverAssets	0.07%
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.15%
Other Expenses	rr_OtherExpensesOverAssets	0.22%
Acquired Fund Fees and Expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%	[2]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.02%	[2]
1 Year	rr_ExpenseExampleYear01	525
3 Years	rr_ExpenseExampleYear03	736
5 Years	rr_ExpenseExampleYear05	964
10 Years	rr_ExpenseExampleYear10	1,620
1 Year	rr_AverageAnnualReturnYear01	(11.91%)
5 Years	rr_AverageAnnualReturnYear05	10.53%
Since Inception	rr_AverageAnnualReturnSinceInception	2.01%
Inception Date	rr_AverageAnnualReturnInceptionDate	Aug. 01, 2006
BlackRock High Yield Municipal Fund | Investor C Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOfferingPrice	1.00%	[3]
Management Fee	rr_ManagementFeesOverAssets	0.54%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	1.00%
Interest Expense	rr_Component1OtherExpensesOverAssets	0.07%
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.17%
Other Expenses	rr_OtherExpensesOverAssets	0.24%
Acquired Fund Fees and Expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%	[2]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.79%	[2]
1 Year	rr_ExpenseExampleYear01	282
3 Years	rr_ExpenseExampleYear03	563
5 Years	rr_ExpenseExampleYear05	970
10 Years	rr_ExpenseExampleYear10	2,105
1 Year	rr_ExpenseExampleNoRedemptionYear01	182
3 Years	rr_ExpenseExampleNoRedemptionYear03	563
5 Years	rr_ExpenseExampleNoRedemptionYear05	970
10 Years	rr_ExpenseExampleNoRedemptionYear10	2,105
1 Year	rr_AverageAnnualReturnYear01	(9.56%)
5 Years	rr_AverageAnnualReturnYear05	10.66%
Since Inception	rr_AverageAnnualReturnSinceInception	1.87%
Inception Date	rr_AverageAnnualReturnInceptionDate	Aug. 01, 2006
BlackRock High Yield Municipal Fund | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none
Management Fee	rr_ManagementFeesOverAssets	0.54%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Interest Expense	rr_Component1OtherExpensesOverAssets	0.07%
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.15%
Other Expenses	rr_OtherExpensesOverAssets	0.22%
Acquired Fund Fees and Expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%	[2]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.77%	[2]
1 Year	rr_ExpenseExampleYear01	79
3 Years	rr_ExpenseExampleYear03	246
5 Years	rr_ExpenseExampleYear05	428
10 Years	rr_ExpenseExampleYear10	$ 954
2007	rr_AnnualReturn2007	(5.88%)
2008	rr_AnnualReturn2008	(27.36%)
2009	rr_AnnualReturn2009	38.76%
2010	rr_AnnualReturn2010	5.60%
2011	rr_AnnualReturn2011	12.13%
2012	rr_AnnualReturn2012	15.24%
2013	rr_AnnualReturn2013	(7.75%)
Year to Date Return, Label	rr_YearToDateReturnLabel	year-to-date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2014
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	14.49%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Sep. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	16.53%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2008
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(19.35%)
1 Year	rr_AverageAnnualReturnYear01	(7.75%)
5 Years	rr_AverageAnnualReturnYear05	11.80%
Since Inception	rr_AverageAnnualReturnSinceInception	2.91%
Inception Date	rr_AverageAnnualReturnInceptionDate	Aug. 01, 2006
BlackRock High Yield Municipal Fund | Return After Taxes on Distributions | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(7.77%)
5 Years	rr_AverageAnnualReturnYear05	11.79%
Since Inception	rr_AverageAnnualReturnSinceInception	2.89%
BlackRock High Yield Municipal Fund | Return After Taxes on Distributions and Sale of Shares | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(2.33%)
5 Years	rr_AverageAnnualReturnYear05	10.76%
Since Inception	rr_AverageAnnualReturnSinceInception	3.36%
BlackRock High Yield Municipal Fund | S&P ® Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(2.55%)
5 Years	rr_AverageAnnualReturnYear05	6.34%
Since Inception	rr_AverageAnnualReturnSinceInception	4.36%
BlackRock High Yield Municipal Fund | S&P ® Customized High Yield Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(4.21%)
5 Years	rr_AverageAnnualReturnYear05	12.31%
Since Inception	rr_AverageAnnualReturnSinceInception	3.83%
BlackRock High Yield Municipal Fund | Custom High Yield Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(4.91%)	[4]
5 Years	rr_AverageAnnualReturnYear05	12.15%	[4]
Since Inception	rr_AverageAnnualReturnSinceInception	3.73%	[4]

[1] A contingent deferred sales charge ("CDSC") of 1.00% is assessed on certain redemptions of Investor A Shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase of an investment of $1,000,000 or more.

[2] The Total Annual Fund Operating Expenses do not correlate to the ratio of expenses to average net assets given in the Fund's most recent annual report, which does not include Acquired Fund Fees and Expenses.

[3] There is no CDSC on Investor C Shares after one year.

[4] The Custom High Yield Index reflects the returns of the S&P® Customized High Yield Municipal Bond Index for periods prior to January 1, 2013, and the returns of only those bonds in the S&P® Customized High Yield Municipal Bond Index that have maturities greater than 5 years for periods subsequent to January 1, 2013.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	BLACKROCK MUNICIPAL BOND FUND, INC.
Prospectus Date	rr_ProspectusDate	Oct. 28, 2014
Document Creation Date	dei_DocumentCreationDate	Oct. 28, 2014

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	BLACKROCK MUNICIPAL BOND FUND, INC.
Prospectus Date	rr_ProspectusDate	Oct. 28, 2014
BlackRock | BlackRock National Municipal Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Overview Key Facts About BlackRock National Municipal Fund
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The investment objective of BlackRock National Municipal Fund (the “National Fund” or the “Fund”) is to provide shareholders with as high a level of income exempt from Federal income taxes as is consistent with the investment policies of the Fund.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold BlackRock Shares of the Fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	November 1, 2015
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover:
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 35% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	35.00%
Expense Example [Heading]	rr_ExpenseExampleHeading	Example:
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies of the Fund
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the National Fund seeks to achieve its objective by investing at least 80% of its assets in municipal bonds. Municipal bonds include debt obligations issued by or on behalf of a governmental entity or other qualifying issuer that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax). Municipal bonds may be obligations of a variety of issuers, including governmental entities or other qualifying issuers. Issuers may be states, territories and possessions of the United States and the District of Columbia and their political subdivisions, agencies and instrumentalities. Municipal bonds also include short-term tax-exempt obligations like municipal notes and variable rate demand obligations. The Fund may invest in municipal bonds rated in any rating category or in unrated municipal bonds. Although Fund management presently intends to invest at least 65% of the Fund's net assets in municipal bonds rated investment grade or in unrated municipal bonds that Fund management believes are of comparable quality, it is possible that in the future the Fund could invest up to 100% of its assets in "junk bonds." Investment grade bonds are securities which are rated in the four highest categories by at least one of the major rating agencies or determined by the management team to be of similar quality. The Fund does not intend to invest more than 35% of its net assets in junk bonds or in unrated bonds that Fund management believes are of comparable quality. This is a non-fundamental policy and may be changed by the Board at any time. The Fund will usually invest in municipal bonds that have a maturity of five years or longer. The Fund may engage in transactions in certain derivatives, such as financial futures contracts and options thereon, indexed and inverse floating rate obligations and swap agreements, including credit default swap agreements. The Fund may use derivative instruments to hedge its investments or to seek to enhance returns. The Fund may leverage its assets through the use of proceeds received through tender option bond transactions. In a tender option bond transaction, a third party sponsor establishes a special purpose entity, the tender option bond trust (a "TOB Trust"), into which the Fund transfers municipal bonds or other municipal securities. The Fund may invest in tender option bonds and residual interest tender option bonds and may also invest in securities the return of which is inversely related to changes in an interest rate ("inverse floaters"). The Fund will look through to the underlying municipal bond held by a TOB Trust for purposes of the Fund's 80% policy. The Fund may invest in tender option bonds on either a non-recourse or recourse basis. If the Fund invests in a TOB Trust on a recourse basis it will bear the risk of loss with respect to any liquidation of the TOB Trust.
Risk [Heading]	rr_RiskHeading	Principal Risks of Investing in the Fund
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund. Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things. Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund's investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund's investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund's net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the "full faith and credit" of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund's performance. Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make principal and interest payments when due. Changes in an issuer's credit rating or the market's perception of an issuer's creditworthiness may also affect the value of the Fund's investment in that issuer. The degree of credit risk depends on the issuer's financial condition and on the terms of the securities. Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall. Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields. Derivatives Risk — The Fund's use of derivatives may reduce the Fund's returns and/or increase volatility. Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Fund's use of derivatives is that the fluctuations in their values may not correlate perfectly with the overall securities markets. Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will not fulfill its contractual obligation. The possible lack of a liquid secondary market for derivatives and the resulting inability of the Fund to sell or otherwise close a derivatives position could expose the Fund to losses and could make derivatives more difficult for the Fund to value accurately. Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them. Derivatives also may expose the Fund to greater risk and increase its costs. Certain transactions in derivatives involve substantial leverage risk and may expose the Fund to potential losses that exceed the amount originally invested by the Fund. The U.S. Government is in the process of adopting and implementing regulations governing derivatives markets, including mandatory clearing of certain derivatives, margin, reporting and registration requirements. The ultimate impact of the regulations remains unclear. Additional U.S. or other regulations may make derivatives more costly, may limit the availability of derivatives, or may otherwise adversely affect the value or performance of derivatives. Certain aspects of the tax treatment of derivative instruments, including swap agreements and commodity-linked derivative instruments are currently unclear and may be affected by changes in legislation, regulations or other legally binding authority that could affect the character, timing and amount of the Fund's taxable income or gains and distributions. Junk Bonds Risk — Although junk bonds generally pay higher rates of interest than investment grade bonds, junk bonds are high risk investments that may cause income and principal losses for the Fund. Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund's portfolio will be magnified when the Fund uses leverage. Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund's investment in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund's principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquid investments may become illiquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions. Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money. Municipal Securities Risks — Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include: General Obligation Bonds Risks — Timely payments depend on the issuer's credit quality, ability to raise tax revenues and ability to maintain an adequate tax base. Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source. Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its faith, credit and taxing power for repayment. Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality. Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money. Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property. Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers' bond counsel and, in the case of derivative securities, sponsors' counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities. Tender Option Bonds and Related Securities Risk — The Fund's use of tender option bonds may reduce the Fund's return and/or increase volatility. Investments in tender option bonds expose the Fund to counterparty risk and leverage risk. An investment in tender option bonds typically will involve greater risk than an investment in a municipal fixed rate security, including the risk of loss of principal. Distributions on residual interest tender option bonds ("TOB Residuals") and inverse floaters will bear an inverse relationship to short-term municipal security interest rates. Distributions on TOB Residuals and inverse floaters paid to the Fund will be reduced or, in the extreme, eliminated as short-term municipal interest rates rise and will increase when short-term municipal interest rates fall. TOB Residuals and inverse floaters generally will underperform the market for fixed rate municipal securities in a rising interest rate environment.
Risk Lose Money [Text]	rr_RiskLoseMoney	You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the S&P® Municipal Bond Index and the Custom National Index. Effective as of September 1, 2014, a customized benchmark, referred to as the “Custom National Index,” was added as a performance benchmark against which the Fund measures its performance. Fund management believes that this performance benchmark more accurately reflects the investment strategy of the Fund. The returns for BlackRock Shares prior to July 18, 2011, the commencement of operations of BlackRock Shares, are based upon performance of the Fund’s Institutional Shares adjusted to reflect the class-specific fees applicable to BlackRock Shares. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com/funds or can be obtained by phone at (800) 882-0052.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund.
Performance Additional Market Index [Text]	rr_PerformanceAdditionalMarketIndex	The table compares the Fund's performance to that of the S&P® Municipal Bond Index and the Custom National Index. Effective as of September 1, 2014, a customized benchmark, referred to as the "Custom National Index," was added as a performance benchmark against which the Fund measures its performance. Fund management believes that this performance benchmark more accurately reflects the investment strategy of the Fund.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	(800) 882-0052
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.blackrock.com/funds
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	As with all such investments, past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading]	rr_BarChartHeading	BlackRock Shares ANNUAL TOTAL RETURNS BlackRock National Municipal Fund As of 12/31
Bar Chart Does Not Reflect Sales Loads [Text]	rr_BarChartDoesNotReflectSalesLoads	Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown.
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	During the ten-year period shown in the bar chart, the highest return for a quarter was 7.64% (quarter ended September 30, 2009) and the lowest return for a quarter was -4.60% (quarter ended December 31, 2010). The year-to-date return as of September 30, 2014 was 9.28%.
Performance Table Heading	rr_PerformanceTableHeading	As of 12/31/13 Average Annual Total Returns
Performance Table Does Reflect Sales Loads	rr_PerformanceTableDoesReflectSalesLoads	However, the table includes all applicable fees and sales charges.
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
BlackRock | BlackRock National Municipal Fund | BlackRock Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none
Management Fee	rr_ManagementFeesOverAssets	0.48%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Interest Expense	rr_Component1OtherExpensesOverAssets	0.06%
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.07%
Other Expenses	rr_OtherExpensesOverAssets	0.13%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.61%
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.09%)	[1]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	0.52%	[1]
1 Year	rr_ExpenseExampleYear01	$ 53
3 Years	rr_ExpenseExampleYear03	186
5 Years	rr_ExpenseExampleYear05	331
10 Years	rr_ExpenseExampleYear10	$ 754
2004	rr_AnnualReturn2004	5.62%
2005	rr_AnnualReturn2005	4.93%
2006	rr_AnnualReturn2006	5.52%
2007	rr_AnnualReturn2007	1.70%
2008	rr_AnnualReturn2008	(8.46%)
2009	rr_AnnualReturn2009	18.01%
2010	rr_AnnualReturn2010	3.22%
2011	rr_AnnualReturn2011	11.68%
2012	rr_AnnualReturn2012	9.92%
2013	rr_AnnualReturn2013	(2.95%)
Year to Date Return, Label	rr_YearToDateReturnLabel	year-to-date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2014
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	9.28%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Sep. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	7.64%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2010
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(4.60%)
1 Year	rr_AverageAnnualReturnYear01	(2.95%)
5 Years	rr_AverageAnnualReturnYear05	7.73%
10 Years	rr_AverageAnnualReturnYear10	4.68%
BlackRock | BlackRock National Municipal Fund | Return After Taxes on Distributions | BlackRock Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(2.95%)
5 Years	rr_AverageAnnualReturnYear05	7.73%
10 Years	rr_AverageAnnualReturnYear10	4.68%
BlackRock | BlackRock National Municipal Fund | Return After Taxes on Distributions and Sale of Fund Shares | BlackRock Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	none
5 Years	rr_AverageAnnualReturnYear05	7.15%
10 Years	rr_AverageAnnualReturnYear10	4.69%
BlackRock | BlackRock National Municipal Fund | S&P ® Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(2.55%)
5 Years	rr_AverageAnnualReturnYear05	6.34%
10 Years	rr_AverageAnnualReturnYear10	4.33%
BlackRock | BlackRock National Municipal Fund | Custom National Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(3.61%)	[2]
5 Years	rr_AverageAnnualReturnYear05	6.11%	[2]
10 Years	rr_AverageAnnualReturnYear10	4.21%	[2]

[1] As described in the "Management of the Funds" section of the Fund's prospectus on page 30, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 0.46% of average daily net assets until November 1, 2015. The contractual agreement may be terminated upon 90 days' notice by a majority of the non-interested directors of the Fund or by a vote of a majority of the outstanding voting securities of the Fund.

[2] The Custom National Index reflects the returns of the S&P® Municipal Bond Index for periods prior to January 1, 2013, and the returns of only those bonds in the S&P® Municipal Bond Index that have maturities greater than 5 years for periods subsequent to January 1, 2013.

XML

Investor C1 | BlackRock National Municipal Fund

Fund Overview

Key Facts About BlackRock National Municipal Fund

Investment Objective

The investment objective of BlackRock National Municipal Fund (the “National Fund” or the “Fund”) is to provide shareholders with as high a level of income exempt from Federal income taxes as is consistent with the investment policies of the Fund.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold Investor C1 Shares of the Fund.

Shareholder Fees
(fees paid directly from your investment)

Shareholder Fees		Investor C1 BlackRock National Municipal Fund Investor C1 Shares
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)		none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	[1]	1.00%

Annual Fund Operating Expenses
(expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses		Investor C1 BlackRock National Municipal Fund Investor C1 Shares
Management Fee		0.48%
Distribution and/or Service (12b-1) Fees		0.80%
Other Expenses		0.14%
Interest Expense		0.06%
Miscellaneous Other Expenses		0.08%
Total Annual Fund Operating Expenses		1.42%
Fee Waivers and/or Expense Reimbursements	[1]	(0.08%)
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	[1]	1.34%

Example:

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example (USD $)	1 Year	3 Years	5 Years	10 Years
Investor C1 BlackRock National Municipal Fund Investor C1 Shares	236	441	769	1,695

You would pay the following expenses if you did not redeem your shares:

Expense Example, No Redemption (USD $)	1 Year	3 Years	5 Years	10 Years
Investor C1 BlackRock National Municipal Fund Investor C1 Shares	136	441	769	1,695

Portfolio Turnover:

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 35% of the average value of its portfolio.

Principal Investment Strategies of the Fund

Under normal circumstances, the National Fund seeks to achieve its objective by investing at least 80% of its assets in municipal bonds. Municipal bonds include debt obligations issued by or on behalf of a governmental entity or other qualifying issuer that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax). Municipal bonds may be obligations of a variety of issuers, including governmental entities or other qualifying issuers. Issuers may be states, territories and possessions of the United States and the District of Columbia and their political subdivisions, agencies and instrumentalities. Municipal bonds also include short-term tax-exempt obligations like municipal notes and variable rate demand obligations.

The Fund may invest in municipal bonds rated in any rating category or in unrated municipal bonds. Although Fund management presently intends to invest at least 65% of the Fund’s net assets in municipal bonds rated investment grade or in unrated municipal bonds that Fund management believes are of comparable quality, it is possible that in the future the Fund could invest up to 100% of its assets in “junk bonds.” Investment grade bonds are securities which are rated in the four highest categories by at least one of the major rating agencies or determined by the management team to be of similar quality. The Fund does not intend to invest more than 35% of its net assets in junk bonds or in unrated bonds that Fund management believes are of comparable quality. This is a non-fundamental policy and may be changed by the Board at any time. The Fund will usually invest in municipal bonds that have a maturity of five years or longer.

The Fund may engage in transactions in certain derivatives, such as financial futures contracts and options thereon, indexed and inverse floating rate obligations and swap agreements, including credit default swap agreements. The Fund may use derivative instruments to hedge its investments or to seek to enhance returns. The Fund may leverage its assets through the use of proceeds received through tender option bond transactions. In a tender option bond transaction, a third party sponsor establishes a special purpose entity, the tender option bond trust (a “TOB Trust”), into which the Fund transfers municipal bonds or other municipal securities. The Fund may invest in tender option bonds and residual interest tender option bonds and may also invest in securities the return of which is inversely related to changes in an interest rate (“inverse floaters”). The Fund will look through to the underlying municipal bond held by a TOB Trust for purposes of the Fund’s 80% policy. The Fund may invest in tender option bonds on either a non-recourse or recourse basis. If the Fund invests in a TOB Trust on a recourse basis it will bear the risk of loss with respect to any liquidation of the TOB Trust.

Principal Risks of Investing in the Fund

Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund.

• Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things.

  Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund's investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund's investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund's net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the "full faith and credit" of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund's performance.
  
  Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make principal and interest payments when due. Changes in an issuer's credit rating or the market's perception of an issuer's creditworthiness may also affect the value of the Fund's investment in that issuer. The degree of credit risk depends on the issuer's financial condition and on the terms of the securities.
  
  Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall.
  
  Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields.

• Derivatives Risk — The Fund's use of derivatives may reduce the Fund's returns and/or increase volatility. Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Fund's use of derivatives is that the fluctuations in their values may not correlate perfectly with the overall securities markets. Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will not fulfill its contractual obligation. The possible lack of a liquid secondary market for derivatives and the resulting inability of the Fund to sell or otherwise close a derivatives position could expose the Fund to losses and could make derivatives more difficult for the Fund to value accurately. Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them. Derivatives also may expose the Fund to greater risk and increase its costs. Certain transactions in derivatives involve substantial leverage risk and may expose the Fund to potential losses that exceed the amount originally invested by the Fund. The U.S. Government is in the process of adopting and implementing regulations governing derivatives markets, including mandatory clearing of certain derivatives, margin, reporting and registration requirements. The ultimate impact of the regulations remains unclear. Additional U.S. or other regulations may make derivatives more costly, may limit the availability of derivatives, or may otherwise adversely affect the value or performance of derivatives. Certain aspects of the tax treatment of derivative instruments, including swap agreements and commodity-linked derivative instruments are currently unclear and may be affected by changes in legislation, regulations or other legally binding authority that could affect the character, timing and amount of the Fund's taxable income or gains and distributions.

• Junk Bonds Risk — Although junk bonds generally pay higher rates of interest than investment grade bonds, junk bonds are high risk investments that may cause income and principal losses for the Fund.

• Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund's portfolio will be magnified when the Fund uses leverage.

• Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund's investment in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund's principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquid investments may become illiquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions.

• Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money.

• Municipal Securities Risks — Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include:

  General Obligation Bonds Risks — Timely payments depend on the issuer's credit quality, ability to raise tax revenues and ability to maintain an adequate tax base.
  
  Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source.
  
  Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its faith, credit and taxing power for repayment.
  
  Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality.
  
  Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money.
  
  Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property.
  
  Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers' bond counsel and, in the case of derivative securities, sponsors' counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities.

• Tender Option Bonds and Related Securities Risk — The Fund's use of tender option bonds may reduce the Fund's return and/or increase volatility. Investments in tender option bonds expose the Fund to counterparty risk and leverage risk. An investment in tender option bonds typically will involve greater risk than an investment in a municipal fixed rate security, including the risk of loss of principal. Distributions on residual interest tender option bonds ("TOB Residuals") and inverse floaters will bear an inverse relationship to short-term municipal security interest rates. Distributions on TOB Residuals and inverse floaters paid to the Fund will be reduced or, in the extreme, eliminated as short-term municipal interest rates rise and will increase when short-term municipal interest rates fall. TOB Residuals and inverse floaters generally will underperform the market for fixed rate municipal securities in a rising interest rate environment.

Performance Information

The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the S&P^® Municipal Bond Index and the Custom National Index. Effective as of September 1, 2014, a customized benchmark, referred to as the “Custom National Index,” was added as a performance benchmark against which the Fund measures its performance. Fund management believes that this performance benchmark more accurately reflects the investment strategy of the Fund. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com/funds or can be obtained by phone at (800) 882-0052.

Investor C1 Shares
ANNUAL TOTAL RETURNS
BlackRock National Municipal Fund
As of 12/31

During the ten-year period shown in the bar chart, the highest return for a quarter was 7.39% (quarter ended September 30, 2009) and the lowest return for a quarter was -4.82% (quarter ended December 31, 2010). The year-to-date return as of September 30, 2014 was 8.51%.

As of 12/31/13
Average Annual Total Returns

Average Annual Total Returns Investor C1 BlackRock National Municipal Fund		1 Year	5 Years	10 Years
Investor C1 Shares		(4.58%)	6.83%	3.76%
Investor C1 Shares Return After Taxes on Distributions		(4.58%)	6.83%	3.76%
Investor C1 Shares Return After Taxes on Distributions and Sale of Fund Shares		(1.28%)	6.23%	3.78%
S&P ® Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)		(2.55%)	6.34%	4.33%
Custom National Index (Reflects no deduction for fees, expenses or taxes)	[1]	(3.61%)	6.11%	4.21%

After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.

[1] There is no CDSC on Investor C1 Shares after one year.

[1] As described in the "Management of the Funds" section of the Fund's prospectus on page 31, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 1.28% (for Investor C1 Shares) of average daily net assets until November 1, 2015. The contractual agreement may be terminated upon 90 days' notice by a majority of the non-interested directors of the Fund or by a vote of a majority of the outstanding voting securities of the Fund.

[1] The Custom National Index reflects the returns of the S&P® Municipal Bond Index for periods prior to January 1, 2013, and the returns of only those bonds in the S&P® Municipal Bond Index that have maturities greater than 5 years for periods subsequent to January 1, 2013.

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BlackRock High Yield Municipal Fund

Fund Overview

Key Facts About BlackRock High Yield Municipal Fund

Investment Objective

The investment objective of BlackRock High Yield Municipal Fund (the “High Yield Fund” or the “Fund”) is to provide shareholders with as high a level of income exempt from Federal income taxes as is consistent with the investment policies of the Fund.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $100,000 in the fund complex advised by BlackRock Advisors, LLC (“BlackRock”) or its affiliates. More information about these and other discounts is available from your financial professional or your selected securities dealer, broker, investment adviser, service provider or industry professional (including BlackRock, The PNC Financial Services Group, Inc. (“PNC”) and their respective affiliates) (each a “Financial Intermediary”) and in the “Details About the Share Classes” section on page 33 of the Fund’s prospectus and in the “Purchase of Shares” section on page II-67 of Part II of the Fund’s Statement of Additional Information.

Shareholder Fees
(fees paid directly from your investment)

Shareholder Fees BlackRock High Yield Municipal Fund	Investor A Shares		Investor C Shares		Institutional Shares
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	4.25%		none		none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	none	[1]	1.00%	[2]	none

Annual Fund Operating Expenses
(expenses that you pay each year as a
percentage of the value of your investment)

Annual Fund Operating Expenses BlackRock High Yield Municipal Fund		Investor A Shares	Investor C Shares	Institutional Shares
Management Fee		0.54%	0.54%	0.54%
Distribution and/or Service (12b-1) Fees		0.25%	1.00%	none
Other Expenses		0.22%	0.24%	0.22%
Interest Expense		0.07%	0.07%	0.07%
Miscellaneous Other Expenses		0.15%	0.17%	0.15%
Acquired Fund Fees and Expenses	[1]	0.01%	0.01%	0.01%
Total Annual Fund Operating Expenses	[1]	1.02%	1.79%	0.77%

Example:

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example BlackRock High Yield Municipal Fund (USD $)	1 Year	3 Years	5 Years	10 Years
Investor A Shares	525	736	964	1,620
Investor C Shares	282	563	970	2,105
Institutional Shares	79	246	428	954

You would pay the following expenses if you did not redeem your shares:

Expense Example, No Redemption (USD $)	1 Year	3 Years	5 Years	10 Years
BlackRock High Yield Municipal Fund Investor C Shares	182	563	970	2,105

Portfolio Turnover:

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 40% of the average value of its portfolio.

Principal Investment Strategies of the Fund

Under normal circumstances, the High Yield Fund seeks to achieve its objective by investing at least 80% of its assets in municipal bonds. Municipal bonds include debt obligations issued by or on behalf of a governmental entity or other qualifying issuer that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax). Municipal bonds may be obligations of a variety of issuers, including governmental entities or other qualifying issuers. Issuers may be states, territories and possessions of the United States and the District of Columbia and their political subdivisions, agencies and instrumentalities. Municipal bonds also include short-term tax-exempt obligations like municipal notes and variable rate demand obligations.

The High Yield Fund may invest in municipal bonds rated in any rating category or in unrated municipal bonds. Although the Fund may invest in municipal bonds in any rating category, Fund management presently intends to invest at least 65% of the Fund's net assets in medium- to low-quality bonds as rated by at least one independent rating agency (BBB or lower by Standard & Poor's ("S&P") or Fitch Ratings, Inc. ("Fitch") or Baa or lower by Moody's Investors Service, Inc. ("Moody's")), or if unrated, judged to be of comparable quality by BlackRock. Obligations rated below BBB or Baa are commonly known as "junk bonds." It is possible that the Fund could invest up to 100% of its assets in "junk bonds."

The Fund may also invest up to 10% of its assets in municipal bonds that are distressed securities. Distressed securities are securities that are the subject of bankruptcy proceedings or otherwise in default as to the repayment of principal and/or payment of interest at the time of acquisition or are rated in the lowest rating categories by at least one independent rating agency (CC or lower by S&P or Fitch or Ca or lower by Moody's), or if unrated, judged to be of comparable quality by BlackRock. The Fund will usually invest in municipal bonds that have a maturity of five years or longer.

Principal Risks of Investing in the Fund

Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund.

• Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things.

  Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund's investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund's investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund's net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the "full faith and credit" of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund's performance.
  
  Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make principal and interest payments when due. Changes in an issuer's credit rating or the market's perception of an issuer's creditworthiness may also affect the value of the Fund's investment in that issuer. The degree of credit risk depends on the issuer's financial condition and on the terms of the securities.
  
  Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall.
  
  Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields.

• Distressed Securities Risk — Distressed securities are speculative and involve substantial risks in addition to the risks of investing in junk bonds. The Fund will generally not receive interest payments on the distressed securities and may incur costs to protect its investment. In addition, distressed securities involve the substantial risk that principal will not be repaid. These securities may present a substantial risk of default or may be in default at the time of investment. The Fund may incur additional expenses to the extent it is required to seek recovery upon a default in the payment of principal of or interest on its portfolio holdings. In any reorganization or liquidation proceeding relating to a portfolio company, the Fund may lose its entire investment or may be required to accept cash or securities with a value less than its original investment. Distressed securities and any securities received in an exchange for such securities may be subject to restrictions on resale.

• Junk Bonds Risk — Although junk bonds generally pay higher rates of interest than investment grade bonds, junk bonds are high risk investments that may cause income and principal losses for the Fund.

• Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund's portfolio will be magnified when the Fund uses leverage.

• Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund's investment in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund's principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquid investments may become illiquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions.

• Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money.

• Municipal Securities Risks — Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include:

  General Obligation Bonds Risks — Timely payments depend on the issuer's credit quality, ability to raise tax revenues and ability to maintain an adequate tax base.
  
  Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source.
  
  Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its faith, credit and taxing power for repayment.
  
  Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality.
  
  Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money.
  
  Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property.
  
  Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers' bond counsel and, in the case of derivative securities, sponsors' counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities.

Performance Information

The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the S&P^® Municipal Bond Index, the S&P^® Customized High Yield Municipal Bond Index, and the Custom High Yield Index. Effective as of January 1, 2014, a customized benchmark, referred to as the “Custom High Yield Index,” replaced the S&P^® Customized High Yield Municipal Bond Index as a performance benchmark against which the Fund measures its performance. Fund management believes that this performance benchmark more accurately reflects the investment strategy of the Fund. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com/funds or can be obtained by phone at (800) 882-0052.

Institutional Shares
ANNUAL TOTAL RETURNS
BlackRock High Yield Municipal Fund
As of 12/31

During the period shown in the bar chart, the highest return for a quarter was 16.53% (quarter ended September 30, 2009) and the lowest return for a quarter was -19.35% (quarter ended December 31, 2008). The year-to-date return as of September 30, 2014 was 14.49%.

As of 12/31/13
Average Annual Total Returns

Average Annual Total Returns BlackRock High Yield Municipal Fund		1 Year	5 Years	Since Inception	Inception Date
Institutional Shares		(7.75%)	11.80%	2.91%	Aug. 01, 2006
Institutional Shares Return After Taxes on Distributions		(7.77%)	11.79%	2.89%
Institutional Shares Return After Taxes on Distributions and Sale of Shares		(2.33%)	10.76%	3.36%
Investor A Shares		(11.91%)	10.53%	2.01%	Aug. 01, 2006
Investor C Shares		(9.56%)	10.66%	1.87%	Aug. 01, 2006
S&P ® Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)		(2.55%)	6.34%	4.36%
S&P ® Customized High Yield Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)		(4.21%)	12.31%	3.83%
Custom High Yield Index (Reflects no deduction for fees, expenses or taxes)	[1]	(4.91%)	12.15%	3.73%

After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. After-tax returns are shown for Institutional Shares only, and the after-tax returns for Investor A and Investor C Shares will vary.

[1] A contingent deferred sales charge ("CDSC") of 1.00% is assessed on certain redemptions of Investor A Shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase of an investment of $1,000,000 or more.

[2] There is no CDSC on Investor C Shares after one year.

[1] The Total Annual Fund Operating Expenses do not correlate to the ratio of expenses to average net assets given in the Fund's most recent annual report, which does not include Acquired Fund Fees and Expenses.

[1] The Custom High Yield Index reflects the returns of the S&P® Customized High Yield Municipal Bond Index for periods prior to January 1, 2013, and the returns of only those bonds in the S&P® Customized High Yield Municipal Bond Index that have maturities greater than 5 years for periods subsequent to January 1, 2013.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	BLACKROCK MUNICIPAL BOND FUND, INC.
Prospectus Date	rr_ProspectusDate	Oct. 28, 2014
Investor A, B, C and Institutional | BlackRock National Municipal Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Overview Key Facts About BlackRock National Municipal Fund
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The investment objective of BlackRock National Municipal Fund (the “National Fund” or the “Fund”) is to provide shareholders with as high a level of income exempt from Federal income taxes as is consistent with the investment policies of the Fund.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $100,000 in the fund complex advised by BlackRock Advisors, LLC (“BlackRock”) or its affiliates. More information about these and other discounts is available from your financial professional or your selected securities dealer, broker, investment adviser, service provider or industry professional (including BlackRock, The PNC Financial Services Group, Inc. (“PNC”) and their respective affiliates) (each a “Financial Intermediary”) and in the “Details About the Share Classes” section on page 33 of the Fund’s prospectus and in the “Purchase of Shares” section on page II-67 of Part II of the Fund’s Statement of Additional Information.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	November 1, 2015
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover:
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 35% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	35.00%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	A contingent deferred sales charge (“CDSC”) of 1.00% is assessed on certain redemptions of Investor A Shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase of an investment of $1,000,000 or more.
Expense Breakpoint Discounts [Text]	rr_ExpenseBreakpointDiscounts	You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $100,000 in the fund complex advised by BlackRock Advisors, LLC (“BlackRock”) or its affiliates.
Expense Breakpoint, Minimum Investment Required [Amount]	rr_ExpenseBreakpointMinimumInvestmentRequiredAmount	$ 100,000
Expense Example [Heading]	rr_ExpenseExampleHeading	Example:
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example, No Redemption, By Year, Caption [Text]	rr_ExpenseExampleNoRedemptionByYearCaption	You would pay the following expenses if you did not redeem your shares:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies of the Fund
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the National Fund seeks to achieve its objective by investing at least 80% of its assets in municipal bonds. Municipal bonds include debt obligations issued by or on behalf of a governmental entity or other qualifying issuer that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax). Municipal bonds may be obligations of a variety of issuers, including governmental entities or other qualifying issuers. Issuers may be states, territories and possessions of the United States and the District of Columbia and their political subdivisions, agencies and instrumentalities. Municipal bonds also include short-term tax-exempt obligations like municipal notes and variable rate demand obligations. The Fund may invest in municipal bonds rated in any rating category or in unrated municipal bonds. Although Fund management presently intends to invest at least 65% of the Fund’s net assets in municipal bonds rated investment grade or in unrated municipal bonds that Fund management believes are of comparable quality, it is possible that in the future the Fund could invest up to 100% of its assets in “junk bonds.” Investment grade bonds are securities which are rated in the four highest categories by at least one of the major rating agencies or determined by the management team to be of similar quality. The Fund does not intend to invest more than 35% of its net assets in junk bonds or in unrated bonds that Fund management believes are of comparable quality. This is a non-fundamental policy and may be changed by the Board at any time. The Fund will usually invest in municipal bonds that have a maturity of five years or longer. The Fund may engage in transactions in certain derivatives, such as financial futures contracts and options thereon, indexed and inverse floating rate obligations and swap agreements, including credit default swap agreements. The Fund may use derivative instruments to hedge its investments or to seek to enhance returns. The Fund may leverage its assets through the use of proceeds received through tender option bond transactions. In a tender option bond transaction, a third party sponsor establishes a special purpose entity, the tender option bond trust (a “TOB Trust”), into which the Fund transfers municipal bonds or other municipal securities. The Fund may invest in tender option bonds and residual interest tender option bonds and may also invest in securities the return of which is inversely related to changes in an interest rate (“inverse floaters”). The Fund will look through to the underlying municipal bond held by a TOB Trust for purposes of the Fund’s 80% policy. The Fund may invest in tender option bonds on either a non-recourse or recourse basis. If the Fund invests in a TOB Trust on a recourse basis it will bear the risk of loss with respect to any liquidation of the TOB Trust.
Risk [Heading]	rr_RiskHeading	Principal Risks of Investing in the Fund
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund. Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things. Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund's investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund's investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund's net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the "full faith and credit" of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund's performance. Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make principal and interest payments when due. Changes in an issuer's credit rating or the market's perception of an issuer's creditworthiness may also affect the value of the Fund's investment in that issuer. The degree of credit risk depends on the issuer's financial condition and on the terms of the securities. Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall. Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields. Derivatives Risk — The Fund's use of derivatives may reduce the Fund's returns and/or increase volatility. Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Fund's use of derivatives is that the fluctuations in their values may not correlate perfectly with the overall securities markets. Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will not fulfill its contractual obligation. The possible lack of a liquid secondary market for derivatives and the resulting inability of the Fund to sell or otherwise close a derivatives position could expose the Fund to losses and could make derivatives more difficult for the Fund to value accurately. Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them. Derivatives also may expose the Fund to greater risk and increase its costs. Certain transactions in derivatives involve substantial leverage risk and may expose the Fund to potential losses that exceed the amount originally invested by the Fund. The U.S. Government is in the process of adopting and implementing regulations governing derivatives markets, including mandatory clearing of certain derivatives, margin, reporting and registration requirements. The ultimate impact of the regulations remains unclear. Additional U.S. or other regulations may make derivatives more costly, may limit the availability of derivatives, or may otherwise adversely affect the value or performance of derivatives. Certain aspects of the tax treatment of derivative instruments, including swap agreements and commodity-linked derivative instruments are currently unclear and may be affected by changes in legislation, regulations or other legally binding authority that could affect the character, timing and amount of the Fund's taxable income or gains and distributions. Junk Bonds Risk — Although junk bonds generally pay higher rates of interest than investment grade bonds, junk bonds are high risk investments that may cause income and principal losses for the Fund. Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund's portfolio will be magnified when the Fund uses leverage. Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund's investment in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund's principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquid investments may become illiquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions. Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money. Municipal Securities Risks — Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include: General Obligation Bonds Risks — Timely payments depend on the issuer's credit quality, ability to raise tax revenues and ability to maintain an adequate tax base. Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source. Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its faith, credit and taxing power for repayment. Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality. Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money. Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property. Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers' bond counsel and, in the case of derivative securities, sponsors' counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities. Tender Option Bonds and Related Securities Risk — The Fund's use of tender option bonds may reduce the Fund's return and/or increase volatility. Investments in tender option bonds expose the Fund to counterparty risk and leverage risk. An investment in tender option bonds typically will involve greater risk than an investment in a municipal fixed rate security, including the risk of loss of principal. Distributions on residual interest tender option bonds ("TOB Residuals") and inverse floaters will bear an inverse relationship to short-term municipal security interest rates. Distributions on TOB Residuals and inverse floaters paid to the Fund will be reduced or, in the extreme, eliminated as short-term municipal interest rates rise and will increase when short-term municipal interest rates fall. TOB Residuals and inverse floaters generally will underperform the market for fixed rate municipal securities in a rising interest rate environment.
Risk Lose Money [Text]	rr_RiskLoseMoney	You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the S&P® Municipal Bond Index and the Custom National Index. Effective as of September 1, 2014, a customized benchmark, referred to as the “Custom National Index,” was added as a performance benchmark against which the Fund measures its performance. Fund management believes that this performance benchmark more accurately reflects the investment strategy of the Fund. Prior to the Investor C Shares inception date of October 2, 2006, the returns for Investor C Shares are based on the performance of the Institutional Shares adjusted to reflect the distribution and service (12b-1) fees applicable to Investor C Shares. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com/funds or can be obtained by phone at (800) 882-0052.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund.
Performance Additional Market Index [Text]	rr_PerformanceAdditionalMarketIndex	The table compares the Fund’s performance to that of the S&P® Municipal Bond Index and the Custom National Index. Effective as of September 1, 2014, a customized benchmark, referred to as the “Custom National Index,” was added as a performance benchmark against which the Fund measures its performance. Fund management believes that this performance benchmark more accurately reflects the investment strategy of the Fund.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	(800) 882-0052
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.blackrock.com/funds
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	As with all such investments, past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading]	rr_BarChartHeading	Investor A Shares ANNUAL TOTAL RETURNS BlackRock National Municipal Fund As of 12/31
Bar Chart Does Not Reflect Sales Loads [Text]	rr_BarChartDoesNotReflectSalesLoads	Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown.
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	During the ten-year period shown in the bar chart, the highest return for a quarter was 7.42% (quarter ended September 30, 2009) and the lowest return for a quarter was -4.66% (quarter ended December 31, 2008). The year-to-date return as of September 30, 2014 was 8.96%.
Performance Table Heading	rr_PerformanceTableHeading	As of 12/31/13 Average Annual Total Returns
Performance Table Does Reflect Sales Loads	rr_PerformanceTableDoesReflectSalesLoads	However, the table includes all applicable fees and sales charges.
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Performance Table One Class of after Tax Shown [Text]	rr_PerformanceTableOneClassOfAfterTaxShown	After-tax returns are shown for Investor A Shares only, and the after-tax returns for Investor B, Investor C and Institutional Shares will vary.
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. After-tax returns are shown for Investor A Shares only, and the after-tax returns for Investor B, Investor C and Institutional Shares will vary.
Investor A, B, C and Institutional | BlackRock National Municipal Fund | Investor A Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	4.25%
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none	[1]
Management Fee	rr_ManagementFeesOverAssets	0.48%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Interest Expense	rr_Component1OtherExpensesOverAssets	0.06%
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.11%
Other Expenses	rr_OtherExpensesOverAssets	0.17%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.90%
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.12%)	[2]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	0.78%	[2]
1 Year	rr_ExpenseExampleYear01	501
3 Years	rr_ExpenseExampleYear03	688
5 Years	rr_ExpenseExampleYear05	891
10 Years	rr_ExpenseExampleYear10	1,475
2004	rr_AnnualReturn2004	5.32%
2005	rr_AnnualReturn2005	4.44%
2006	rr_AnnualReturn2006	5.22%
2007	rr_AnnualReturn2007	1.34%
2008	rr_AnnualReturn2008	(8.87%)
2009	rr_AnnualReturn2009	17.56%
2010	rr_AnnualReturn2010	2.94%
2011	rr_AnnualReturn2011	11.36%
2012	rr_AnnualReturn2012	9.63%
2013	rr_AnnualReturn2013	(3.19%)
Year to Date Return, Label	rr_YearToDateReturnLabel	year-to-date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2014
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	8.96%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Sep. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	7.42%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2008
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(4.66%)
1 Year	rr_AverageAnnualReturnYear01	(7.31%)
5 Years	rr_AverageAnnualReturnYear05	6.49%
10 Years	rr_AverageAnnualReturnYear10	3.88%
Investor A, B, C and Institutional | BlackRock National Municipal Fund | Investor B Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOfferingPrice	4.00%	[3]
Management Fee	rr_ManagementFeesOverAssets	0.48%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.75%
Interest Expense	rr_Component1OtherExpensesOverAssets	0.06%
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.11%
Other Expenses	rr_OtherExpensesOverAssets	0.17%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.40%
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.11%)	[2]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	1.29%	[2]
1 Year	rr_ExpenseExampleYear01	531
3 Years	rr_ExpenseExampleYear03	732
5 Years	rr_ExpenseExampleYear05	955
10 Years	rr_ExpenseExampleYear10	1,670
1 Year	rr_ExpenseExampleNoRedemptionYear01	131
3 Years	rr_ExpenseExampleNoRedemptionYear03	432
5 Years	rr_ExpenseExampleNoRedemptionYear05	755
10 Years	rr_ExpenseExampleNoRedemptionYear10	1,670
1 Year	rr_AverageAnnualReturnYear01	(7.42%)
5 Years	rr_AverageAnnualReturnYear05	6.57%
10 Years	rr_AverageAnnualReturnYear10	3.81%
Investor A, B, C and Institutional | BlackRock National Municipal Fund | Investor C Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOfferingPrice	1.00%	[4]
Management Fee	rr_ManagementFeesOverAssets	0.48%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	1.00%
Interest Expense	rr_Component1OtherExpensesOverAssets	0.06%
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.08%
Other Expenses	rr_OtherExpensesOverAssets	0.14%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.62%
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.09%)	[2]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	1.53%	[2]
1 Year	rr_ExpenseExampleYear01	256
3 Years	rr_ExpenseExampleYear03	502
5 Years	rr_ExpenseExampleYear05	873
10 Years	rr_ExpenseExampleYear10	1,914
1 Year	rr_ExpenseExampleNoRedemptionYear01	156
3 Years	rr_ExpenseExampleNoRedemptionYear03	502
5 Years	rr_ExpenseExampleNoRedemptionYear05	873
10 Years	rr_ExpenseExampleNoRedemptionYear10	1,914
1 Year	rr_AverageAnnualReturnYear01	(4.85%)
5 Years	rr_AverageAnnualReturnYear05	6.63%
10 Years	rr_AverageAnnualReturnYear10	3.54%
Investor A, B, C and Institutional | BlackRock National Municipal Fund | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none
Management Fee	rr_ManagementFeesOverAssets	0.48%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Interest Expense	rr_Component1OtherExpensesOverAssets	0.06%
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.14%
Other Expenses	rr_OtherExpensesOverAssets	0.20%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.68%
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets		[2]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	0.68%	[2]
1 Year	rr_ExpenseExampleYear01	69
3 Years	rr_ExpenseExampleYear03	218
5 Years	rr_ExpenseExampleYear05	379
10 Years	rr_ExpenseExampleYear10	$ 847
1 Year	rr_AverageAnnualReturnYear01	(3.06%)
5 Years	rr_AverageAnnualReturnYear05	7.61%
10 Years	rr_AverageAnnualReturnYear10	4.55%
Investor A, B, C and Institutional | BlackRock National Municipal Fund | Return After Taxes on Distributions | Investor A Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(7.31%)
5 Years	rr_AverageAnnualReturnYear05	6.49%
10 Years	rr_AverageAnnualReturnYear10	3.88%
Investor A, B, C and Institutional | BlackRock National Municipal Fund | Return After Taxes on Distributions and Sale of Fund Shares | Investor A Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(2.65%)
5 Years	rr_AverageAnnualReturnYear05	6.07%
10 Years	rr_AverageAnnualReturnYear10	3.98%
Investor A, B, C and Institutional | BlackRock National Municipal Fund | S&P ® Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(2.55%)
5 Years	rr_AverageAnnualReturnYear05	6.34%
10 Years	rr_AverageAnnualReturnYear10	4.33%
Investor A, B, C and Institutional | BlackRock National Municipal Fund | Custom National Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(3.61%)	[5]
5 Years	rr_AverageAnnualReturnYear05	6.11%	[5]
10 Years	rr_AverageAnnualReturnYear10	4.21%	[5]

[1] A contingent deferred sales charge ("CDSC") of 1.00% is assessed on certain redemptions of Investor A Shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase of an investment of $1,000,000 or more.

[2] As described in the "Management of the Funds" section of the Fund's prospectus on page 48, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 0.72% (for Investor A Shares), 1.23% (for Investor B Shares) and 1.47% (for Investor C Shares) of average daily net assets until November 1, 2015. The contractual agreement may be terminated upon 90 days' notice by a majority of the non-interested directors of the Fund or by a vote of a majority of the outstanding voting securities of the Fund.

[3] The CDSC is 4.00% if shares are redeemed in less than two years. The CDSC for Investor B Shares decreases for redemptions made in subsequent years. After six years there is no CDSC on Investor B Shares. (See the section "Details About the Share Classes — Investor B Shares" in the Fund's prospectus for the complete schedule of CDSCs.)

[4] There is no CDSC on Investor C Shares after one year.

[5] The Custom National Index reflects the returns of the S&P® Municipal Bond Index for periods prior to January 1, 2013, and the returns of only those bonds in the S&P® Municipal Bond Index that have maturities greater than 5 years for periods subsequent to January 1, 2013.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	Jun. 30, 2014
Registrant Name	dei_EntityRegistrantName	BLACKROCK MUNICIPAL BOND FUND, INC.
Central Index Key	dei_EntityCentralIndexKey	0000225635
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Oct. 28, 2014
Document Effective Date	dei_DocumentEffectiveDate	Oct. 28, 2014
Prospectus Date	rr_ProspectusDate	Oct. 28, 2014

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Investor A, C and Institutional | BlackRock Short-Term Municipal Fund

Fund Overview

Key Facts About BlackRock Short-Term Municipal Fund

Investment Objective

The investment objective of BlackRock Short-Term Municipal Fund (the “Short-Term Fund” or the “Fund”) is to provide shareholders with as high a level of income exempt from Federal income taxes as is consistent with the investment policies of the Fund.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the fund complex advised by BlackRock Advisors, LLC (“BlackRock”) or its affiliates. More information about these and other discounts is available from your financial professional or your selected securities dealer, broker, investment adviser, service provider or industry professional (including BlackRock, The PNC Financial Services Group, Inc. (“PNC”) and their respective affiliates) (each a “Financial Intermediary”) and in the “Details About the Share Classes” section on page 33 of the Fund’s prospectus and in the “Purchase of Shares” section on page II-67 of Part II of the Fund’s Statement of Additional Information.

Shareholder Fees
(fees paid directly from your investment)

Shareholder Fees Investor A, C and Institutional BlackRock Short-Term Municipal Fund	Investor A Shares		Investor C Shares		Institutional Shares
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	3.00%		none		none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	none	[1]	1.00%	[2]	none

Annual Fund Operating Expenses
(expenses that you pay each year as a
percentage of the value of your investment)

Annual Fund Operating Expenses Investor A, C and Institutional BlackRock Short-Term Municipal Fund		Investor A Shares	Investor C Shares	Institutional Shares
Management Fee		0.33%	0.33%	0.33%
Distribution and/or Service (12b-1) Fees		0.25%	1.00%	none
Other Expenses		0.09%	0.12%	0.18%
Total Annual Fund Operating Expenses		0.67%	1.45%	0.51%
Fee Waivers and/or Expense Reimbursements	[1]	none	none	(0.11%)
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	[1]	0.67%	1.45%	0.40%

Example:

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example Investor A, C and Institutional BlackRock Short-Term Municipal Fund (USD $)	1 Year	3 Years	5 Years	10 Years
Investor A Shares	366	508	662	1,109
Investor C Shares	248	459	792	1,735
Institutional Shares	41	152	274	630

You would pay the following expenses if you did not redeem your shares:

Expense Example, No Redemption (USD $)	1 Year	3 Years	5 Years	10 Years
Investor A, C and Institutional BlackRock Short-Term Municipal Fund Investor C Shares	148	459	792	1,735

Portfolio Turnover:

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 56% of the average value of its portfolio.

Principal Investment Strategies of the Fund

Under normal circumstances, the Short-Term Fund seeks to achieve its objective by investing at least 80% of its assets in municipal bonds. Municipal bonds include debt obligations issued by or on behalf of a governmental entity or other qualifying issuer that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax). Municipal bonds may be obligations of a variety of issuers, including governmental entities or other qualifying issuers. Issuers may be states, territories and possessions of the United States and the District of Columbia and their political subdivisions, agencies and instrumentalities. Municipal bonds also include short-term tax-exempt obligations like municipal notes and variable rate demand obligations.

The Short-Term Fund invests primarily in investment grade municipal bonds or municipal notes, including variable rate demand obligations. Investment grade bonds are securities which are rated in the four highest categories by at least one of the major rating agencies or determined by the management team to be of similar quality. Generally, the higher the rating of a bond, the higher the likelihood that interest and principal payments will be made on time. Municipal notes are shorter-term municipal debt obligations that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax) and that have a maturity that is generally one year or less.

The Fund will maintain a dollar weighted maturity of no more than two years. Because of their shorter maturities, the Fund's investments will not usually be as sensitive to changes in prevailing interest rates as are long-term municipal bonds. Fluctuations in interest rates on short-term municipal bonds may, however, vary more widely from time to time than those on long-term municipal bonds.

Principal Risks of Investing in the Fund

Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund.

• Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things.

  Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund's investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund's investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund's net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the "full faith and credit" of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund's performance.
  
  Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make principal and interest payments when due. Changes in an issuer's credit rating or the market's perception of an issuer's creditworthiness may also affect the value of the Fund's investment in that issuer. The degree of credit risk depends on the issuer's financial condition and on the terms of the securities.
  
  Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall.
  
  Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields.

• Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund's portfolio will be magnified when the Fund uses leverage.

• Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund's investment in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund's principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquid investments may become illiquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions.

• Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money.

• Municipal Securities Risks — Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include:

  General Obligation Bonds Risks — Timely payments depend on the issuer's credit quality, ability to raise tax revenues and ability to maintain an adequate tax base.
  
  Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source.
  
  Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its faith, credit and taxing power for repayment.
  
  Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality.
  
  Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money.
  
  Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property.
  
  Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers' bond counsel and, in the case of derivative securities, sponsors' counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities.

• Variable Rate Demand Obligations Risk — Variable rate demand obligations are floating rate securities that combine an interest in a long-term municipal bond with a right to demand payment before maturity from a bank or other financial institution. If the bank or financial institution is unable to pay, the Fund may lose money.

Performance Information

The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the S&P^® Municipal Bond Index and the S&P^® Limited Maturity Municipal Bond Index, which are relevant to the Fund because they have characteristics similar to the Fund’s investment strategies. The returns for Investor A and Investor C Shares prior to October 2, 2006, the commencement of operations of Investor A and Investor C Shares, are based upon the performance of the Institutional Shares, as adjusted to reflect the distribution and service (12b-1) fees applicable to Investor A and Investor C Shares. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com/funds or can be obtained by phone at (800) 882-0052.

Institutional Shares
ANNUAL TOTAL RETURNS
BlackRock Short-Term Municipal Fund
As of 12/31

During the ten-year period shown in the bar chart, the highest return for a quarter was 1.67% (quarter ended December 31, 2008) and the lowest return for a quarter was -0.37% (quarter ended June 30, 2004). The year-to-date return as of September 30, 2014 was 0.35%.

As of 12/31/13
Average Annual Total Returns

Average Annual Total Returns Investor A, C and Institutional BlackRock Short-Term Municipal Fund	1 Year	5 Years	10 Years
Institutional Shares	0.58%	1.45%	1.99%
Institutional Shares Return After Taxes on Distributions	0.58%	1.45%	1.99%
Institutional Shares Return After Taxes on Distributions and Sale of Fund Shares	0.58%	1.39%	1.98%
Investor A Shares	(2.80%)	0.56%	1.42%
Investor C Shares	(1.59%)	0.39%	0.95%
S&P ® Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)	(2.55%)	6.34%	4.33%
S&P ® Limited Maturity Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)	1.02%	2.18%	2.68%

After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. After-tax returns are shown for Institutional Shares only, and the after-tax returns for Investor A and Investor C Shares will vary.

[1] A contingent deferred sales charge ("CDSC") of 0.50% is assessed on certain redemptions of Investor A Shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase of an investment of $250,000 or more.

[2] There is no CDSC on Investor C Shares after one year.

[1] As described in the "Management of the Funds" section of the Fund's prospectus on page 48, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 0.69% (for Investor A Shares), 1.55% (for Investor C Shares) and 0.40% (for Institutional Shares) of average daily net assets until November 1, 2015. The contractual agreement may be terminated upon 90 days' notice by a majority of the non-interested trustees of the Fund or by a vote of a majority of the outstanding securities of the Fund.

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BlackRock | BlackRock National Municipal Fund

Fund Overview

Key Facts About BlackRock National Municipal Fund

Investment Objective

The investment objective of BlackRock National Municipal Fund (the “National Fund” or the “Fund”) is to provide shareholders with as high a level of income exempt from Federal income taxes as is consistent with the investment policies of the Fund.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold BlackRock Shares of the Fund.

Shareholder Fees
(fees paid directly from your investment)

Shareholder Fees	BlackRock BlackRock National Municipal Fund BlackRock Shares
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	none

Annual Fund Operating Expenses
(expenses that you pay each year as a
percentage of the value of your investment)

Annual Fund Operating Expenses		BlackRock BlackRock National Municipal Fund BlackRock Shares
Management Fee		0.48%
Distribution and/or Service (12b-1) Fees		none
Other Expenses		0.13%
Interest Expense		0.06%
Miscellaneous Other Expenses		0.07%
Total Annual Fund Operating Expenses		0.61%
Fee Waivers and/or Expense Reimbursements	[1]	(0.09%)
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	[1]	0.52%

Example:

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example (USD $)	1 Year	3 Years	5 Years	10 Years
BlackRock BlackRock National Municipal Fund BlackRock Shares	53	186	331	754

Portfolio Turnover:

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 35% of the average value of its portfolio.

Principal Investment Strategies of the Fund

Under normal circumstances, the National Fund seeks to achieve its objective by investing at least 80% of its assets in municipal bonds. Municipal bonds include debt obligations issued by or on behalf of a governmental entity or other qualifying issuer that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax). Municipal bonds may be obligations of a variety of issuers, including governmental entities or other qualifying issuers. Issuers may be states, territories and possessions of the United States and the District of Columbia and their political subdivisions, agencies and instrumentalities. Municipal bonds also include short-term tax-exempt obligations like municipal notes and variable rate demand obligations.

The Fund may invest in municipal bonds rated in any rating category or in unrated municipal bonds. Although Fund management presently intends to invest at least 65% of the Fund's net assets in municipal bonds rated investment grade or in unrated municipal bonds that Fund management believes are of comparable quality, it is possible that in the future the Fund could invest up to 100% of its assets in "junk bonds." Investment grade bonds are securities which are rated in the four highest categories by at least one of the major rating agencies or determined by the management team to be of similar quality. The Fund does not intend to invest more than 35% of its net assets in junk bonds or in unrated bonds that Fund management believes are of comparable quality. This is a non-fundamental policy and may be changed by the Board at any time. The Fund will usually invest in municipal bonds that have a maturity of five years or longer.

The Fund may engage in transactions in certain derivatives, such as financial futures contracts and options thereon, indexed and inverse floating rate obligations and swap agreements, including credit default swap agreements. The Fund may use derivative instruments to hedge its investments or to seek to enhance returns. The Fund may leverage its assets through the use of proceeds received through tender option bond transactions. In a tender option bond transaction, a third party sponsor establishes a special purpose entity, the tender option bond trust (a "TOB Trust"), into which the Fund transfers municipal bonds or other municipal securities. The Fund may invest in tender option bonds and residual interest tender option bonds and may also invest in securities the return of which is inversely related to changes in an interest rate ("inverse floaters"). The Fund will look through to the underlying municipal bond held by a TOB Trust for purposes of the Fund's 80% policy. The Fund may invest in tender option bonds on either a non-recourse or recourse basis. If the Fund invests in a TOB Trust on a recourse basis it will bear the risk of loss with respect to any liquidation of the TOB Trust.

Principal Risks of Investing in the Fund

Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund.

• Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things.

  Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund's investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund's investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund's net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the "full faith and credit" of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund's performance.
  
  Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make principal and interest payments when due. Changes in an issuer's credit rating or the market's perception of an issuer's creditworthiness may also affect the value of the Fund's investment in that issuer. The degree of credit risk depends on the issuer's financial condition and on the terms of the securities.
  
  Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall.
  
  Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields.

• Derivatives Risk — The Fund's use of derivatives may reduce the Fund's returns and/or increase volatility. Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Fund's use of derivatives is that the fluctuations in their values may not correlate perfectly with the overall securities markets. Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will not fulfill its contractual obligation. The possible lack of a liquid secondary market for derivatives and the resulting inability of the Fund to sell or otherwise close a derivatives position could expose the Fund to losses and could make derivatives more difficult for the Fund to value accurately. Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them. Derivatives also may expose the Fund to greater risk and increase its costs. Certain transactions in derivatives involve substantial leverage risk and may expose the Fund to potential losses that exceed the amount originally invested by the Fund. The U.S. Government is in the process of adopting and implementing regulations governing derivatives markets, including mandatory clearing of certain derivatives, margin, reporting and registration requirements. The ultimate impact of the regulations remains unclear. Additional U.S. or other regulations may make derivatives more costly, may limit the availability of derivatives, or may otherwise adversely affect the value or performance of derivatives. Certain aspects of the tax treatment of derivative instruments, including swap agreements and commodity-linked derivative instruments are currently unclear and may be affected by changes in legislation, regulations or other legally binding authority that could affect the character, timing and amount of the Fund's taxable income or gains and distributions.

• Junk Bonds Risk — Although junk bonds generally pay higher rates of interest than investment grade bonds, junk bonds are high risk investments that may cause income and principal losses for the Fund.

• Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund's portfolio will be magnified when the Fund uses leverage.

• Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund's investment in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund's principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquid investments may become illiquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions.

• Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money.

• Municipal Securities Risks — Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include:

  General Obligation Bonds Risks — Timely payments depend on the issuer's credit quality, ability to raise tax revenues and ability to maintain an adequate tax base.
  
  Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source.
  
  Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its faith, credit and taxing power for repayment.
  
  Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality.
  
  Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money.
  
  Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property.
  
  Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers' bond counsel and, in the case of derivative securities, sponsors' counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities.

• Tender Option Bonds and Related Securities Risk — The Fund's use of tender option bonds may reduce the Fund's return and/or increase volatility. Investments in tender option bonds expose the Fund to counterparty risk and leverage risk. An investment in tender option bonds typically will involve greater risk than an investment in a municipal fixed rate security, including the risk of loss of principal. Distributions on residual interest tender option bonds ("TOB Residuals") and inverse floaters will bear an inverse relationship to short-term municipal security interest rates. Distributions on TOB Residuals and inverse floaters paid to the Fund will be reduced or, in the extreme, eliminated as short-term municipal interest rates rise and will increase when short-term municipal interest rates fall. TOB Residuals and inverse floaters generally will underperform the market for fixed rate municipal securities in a rising interest rate environment.

Performance Information

The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the S&P^® Municipal Bond Index and the Custom National Index. Effective as of September 1, 2014, a customized benchmark, referred to as the “Custom National Index,” was added as a performance benchmark against which the Fund measures its performance. Fund management believes that this performance benchmark more accurately reflects the investment strategy of the Fund. The returns for BlackRock Shares prior to July 18, 2011, the commencement of operations of BlackRock Shares, are based upon performance of the Fund’s Institutional Shares adjusted to reflect the class-specific fees applicable to BlackRock Shares. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com/funds or can be obtained by phone at (800) 882-0052.

BlackRock Shares
ANNUAL TOTAL RETURNS
BlackRock National Municipal Fund
As of 12/31

During the ten-year period shown in the bar chart, the highest return for a quarter was 7.64% (quarter ended September 30, 2009) and the lowest return for a quarter was -4.60% (quarter ended December 31, 2010). The year-to-date return as of September 30, 2014 was 9.28%.

As of 12/31/13
Average Annual Total Returns

Average Annual Total Returns BlackRock BlackRock National Municipal Fund		1 Year	5 Years	10 Years
BlackRock Shares		(2.95%)	7.73%	4.68%
BlackRock Shares Return After Taxes on Distributions		(2.95%)	7.73%	4.68%
BlackRock Shares Return After Taxes on Distributions and Sale of Fund Shares		none	7.15%	4.69%
S&P ® Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)		(2.55%)	6.34%	4.33%
Custom National Index (Reflects no deduction for fees, expenses or taxes)	[1]	(3.61%)	6.11%	4.21%

After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.

[1] As described in the "Management of the Funds" section of the Fund's prospectus on page 30, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 0.46% of average daily net assets until November 1, 2015. The contractual agreement may be terminated upon 90 days' notice by a majority of the non-interested directors of the Fund or by a vote of a majority of the outstanding voting securities of the Fund.

[1] The Custom National Index reflects the returns of the S&P® Municipal Bond Index for periods prior to January 1, 2013, and the returns of only those bonds in the S&P® Municipal Bond Index that have maturities greater than 5 years for periods subsequent to January 1, 2013.

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Investor A, B, C and Institutional | BlackRock National Municipal Fund

Fund Overview

Key Facts About BlackRock National Municipal Fund

Investment Objective

The investment objective of BlackRock National Municipal Fund (the “National Fund” or the “Fund”) is to provide shareholders with as high a level of income exempt from Federal income taxes as is consistent with the investment policies of the Fund.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $100,000 in the fund complex advised by BlackRock Advisors, LLC (“BlackRock”) or its affiliates. More information about these and other discounts is available from your financial professional or your selected securities dealer, broker, investment adviser, service provider or industry professional (including BlackRock, The PNC Financial Services Group, Inc. (“PNC”) and their respective affiliates) (each a “Financial Intermediary”) and in the “Details About the Share Classes” section on page 33 of the Fund’s prospectus and in the “Purchase of Shares” section on page II-67 of Part II of the Fund’s Statement of Additional Information.

Shareholder Fees
(fees paid directly from your investment)

Shareholder Fees Investor A, B, C and Institutional BlackRock National Municipal Fund	Investor A Shares		Investor B Shares		Investor C Shares		Institutional Shares
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	4.25%		none		none		none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	none	[1]	4.00%	[2]	1.00%	[3]	none

Annual Fund Operating Expenses
(expenses that you pay each year as a
percentage of the value of your investment)

Annual Fund Operating Expenses Investor A, B, C and Institutional BlackRock National Municipal Fund		Investor A Shares	Investor B Shares	Investor C Shares	Institutional Shares
Management Fee		0.48%	0.48%	0.48%	0.48%
Distribution and/or Service (12b-1) Fees		0.25%	0.75%	1.00%	none
Other Expenses		0.17%	0.17%	0.14%	0.20%
Interest Expense		0.06%	0.06%	0.06%	0.06%
Miscellaneous Other Expenses		0.11%	0.11%	0.08%	0.14%
Total Annual Fund Operating Expenses		0.90%	1.40%	1.62%	0.68%
Fee Waivers and/or Expense Reimbursements	[1]	(0.12%)	(0.11%)	(0.09%)
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	[1]	0.78%	1.29%	1.53%	0.68%

Example:

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example Investor A, B, C and Institutional BlackRock National Municipal Fund (USD $)	1 Year	3 Years	5 Years	10 Years
Investor A Shares	501	688	891	1,475
Investor B Shares	531	732	955	1,670
Investor C Shares	256	502	873	1,914
Institutional Shares	69	218	379	847

You would pay the following expenses if you did not redeem your shares:

Expense Example, No Redemption Investor A, B, C and Institutional BlackRock National Municipal Fund (USD $)	1 Year	3 Years	5 Years	10 Years
Investor B Shares	131	432	755	1,670
Investor C Shares	156	502	873	1,914

Portfolio Turnover:

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 35% of the average value of its portfolio.

Principal Investment Strategies of the Fund

Under normal circumstances, the National Fund seeks to achieve its objective by investing at least 80% of its assets in municipal bonds. Municipal bonds include debt obligations issued by or on behalf of a governmental entity or other qualifying issuer that pay interest that is, in the opinion of bond counsel to the issuer, generally excludable from gross income for Federal income tax purposes (except that the interest may be includable in taxable income for purposes of the Federal alternative minimum tax). Municipal bonds may be obligations of a variety of issuers, including governmental entities or other qualifying issuers. Issuers may be states, territories and possessions of the United States and the District of Columbia and their political subdivisions, agencies and instrumentalities. Municipal bonds also include short-term tax-exempt obligations like municipal notes and variable rate demand obligations.

The Fund may invest in municipal bonds rated in any rating category or in unrated municipal bonds. Although Fund management presently intends to invest at least 65% of the Fund’s net assets in municipal bonds rated investment grade or in unrated municipal bonds that Fund management believes are of comparable quality, it is possible that in the future the Fund could invest up to 100% of its assets in “junk bonds.” Investment grade bonds are securities which are rated in the four highest categories by at least one of the major rating agencies or determined by the management team to be of similar quality. The Fund does not intend to invest more than 35% of its net assets in junk bonds or in unrated bonds that Fund management believes are of comparable quality. This is a non-fundamental policy and may be changed by the Board at any time. The Fund will usually invest in municipal bonds that have a maturity of five years or longer.

The Fund may engage in transactions in certain derivatives, such as financial futures contracts and options thereon, indexed and inverse floating rate obligations and swap agreements, including credit default swap agreements. The Fund may use derivative instruments to hedge its investments or to seek to enhance returns. The Fund may leverage its assets through the use of proceeds received through tender option bond transactions. In a tender option bond transaction, a third party sponsor establishes a special purpose entity, the tender option bond trust (a “TOB Trust”), into which the Fund transfers municipal bonds or other municipal securities. The Fund may invest in tender option bonds and residual interest tender option bonds and may also invest in securities the return of which is inversely related to changes in an interest rate (“inverse floaters”). The Fund will look through to the underlying municipal bond held by a TOB Trust for purposes of the Fund’s 80% policy. The Fund may invest in tender option bonds on either a non-recourse or recourse basis. If the Fund invests in a TOB Trust on a recourse basis it will bear the risk of loss with respect to any liquidation of the TOB Trust.

Principal Risks of Investing in the Fund

Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund.

• Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things.

  Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund's investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund's investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund's net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the "full faith and credit" of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund's performance.
  
  Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make principal and interest payments when due. Changes in an issuer's credit rating or the market's perception of an issuer's creditworthiness may also affect the value of the Fund's investment in that issuer. The degree of credit risk depends on the issuer's financial condition and on the terms of the securities.
  
  Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall.
  
  Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields.

• Derivatives Risk — The Fund's use of derivatives may reduce the Fund's returns and/or increase volatility. Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Fund's use of derivatives is that the fluctuations in their values may not correlate perfectly with the overall securities markets. Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will not fulfill its contractual obligation. The possible lack of a liquid secondary market for derivatives and the resulting inability of the Fund to sell or otherwise close a derivatives position could expose the Fund to losses and could make derivatives more difficult for the Fund to value accurately. Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them. Derivatives also may expose the Fund to greater risk and increase its costs. Certain transactions in derivatives involve substantial leverage risk and may expose the Fund to potential losses that exceed the amount originally invested by the Fund. The U.S. Government is in the process of adopting and implementing regulations governing derivatives markets, including mandatory clearing of certain derivatives, margin, reporting and registration requirements. The ultimate impact of the regulations remains unclear. Additional U.S. or other regulations may make derivatives more costly, may limit the availability of derivatives, or may otherwise adversely affect the value or performance of derivatives. Certain aspects of the tax treatment of derivative instruments, including swap agreements and commodity-linked derivative instruments are currently unclear and may be affected by changes in legislation, regulations or other legally binding authority that could affect the character, timing and amount of the Fund's taxable income or gains and distributions.

• Junk Bonds Risk — Although junk bonds generally pay higher rates of interest than investment grade bonds, junk bonds are high risk investments that may cause income and principal losses for the Fund.

• Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund's portfolio will be magnified when the Fund uses leverage.

• Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund's investment in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund's principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquid investments may become illiquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions.

• Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money.

• Municipal Securities Risks — Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include:

  General Obligation Bonds Risks — Timely payments depend on the issuer's credit quality, ability to raise tax revenues and ability to maintain an adequate tax base.
  
  Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source.
  
  Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its faith, credit and taxing power for repayment.
  
  Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality.
  
  Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money.
  
  Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property.
  
  Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers' bond counsel and, in the case of derivative securities, sponsors' counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities.

• Tender Option Bonds and Related Securities Risk — The Fund's use of tender option bonds may reduce the Fund's return and/or increase volatility. Investments in tender option bonds expose the Fund to counterparty risk and leverage risk. An investment in tender option bonds typically will involve greater risk than an investment in a municipal fixed rate security, including the risk of loss of principal. Distributions on residual interest tender option bonds ("TOB Residuals") and inverse floaters will bear an inverse relationship to short-term municipal security interest rates. Distributions on TOB Residuals and inverse floaters paid to the Fund will be reduced or, in the extreme, eliminated as short-term municipal interest rates rise and will increase when short-term municipal interest rates fall. TOB Residuals and inverse floaters generally will underperform the market for fixed rate municipal securities in a rising interest rate environment.

Performance Information

The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the S&P^® Municipal Bond Index and the Custom National Index. Effective as of September 1, 2014, a customized benchmark, referred to as the “Custom National Index,” was added as a performance benchmark against which the Fund measures its performance. Fund management believes that this performance benchmark more accurately reflects the investment strategy of the Fund. Prior to the Investor C Shares inception date of October 2, 2006, the returns for Investor C Shares are based on the performance of the Institutional Shares adjusted to reflect the distribution and service (12b-1) fees applicable to Investor C Shares. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com/funds or can be obtained by phone at (800) 882-0052.

Investor A Shares
ANNUAL TOTAL RETURNS
BlackRock National Municipal Fund
As of 12/31

During the ten-year period shown in the bar chart, the highest return for a quarter was 7.42% (quarter ended September 30, 2009) and the lowest return for a quarter was -4.66% (quarter ended December 31, 2008). The year-to-date return as of September 30, 2014 was 8.96%.

As of 12/31/13
Average Annual Total Returns

Average Annual Total Returns Investor A, B, C and Institutional BlackRock National Municipal Fund		1 Year	5 Years	10 Years
Investor A Shares		(7.31%)	6.49%	3.88%
Investor A Shares Return After Taxes on Distributions		(7.31%)	6.49%	3.88%
Investor A Shares Return After Taxes on Distributions and Sale of Fund Shares		(2.65%)	6.07%	3.98%
Investor B Shares		(7.42%)	6.57%	3.81%
Investor C Shares		(4.85%)	6.63%	3.54%
Institutional Shares		(3.06%)	7.61%	4.55%
S&P ® Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)		(2.55%)	6.34%	4.33%
Custom National Index (Reflects no deduction for fees, expenses or taxes)	[1]	(3.61%)	6.11%	4.21%

After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. After-tax returns are shown for Investor A Shares only, and the after-tax returns for Investor B, Investor C and Institutional Shares will vary.

[1] A contingent deferred sales charge ("CDSC") of 1.00% is assessed on certain redemptions of Investor A Shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase of an investment of $1,000,000 or more.

[2] The CDSC is 4.00% if shares are redeemed in less than two years. The CDSC for Investor B Shares decreases for redemptions made in subsequent years. After six years there is no CDSC on Investor B Shares. (See the section "Details About the Share Classes — Investor B Shares" in the Fund's prospectus for the complete schedule of CDSCs.)

[3] There is no CDSC on Investor C Shares after one year.

[1] As described in the "Management of the Funds" section of the Fund's prospectus on page 48, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 0.72% (for Investor A Shares), 1.23% (for Investor B Shares) and 1.47% (for Investor C Shares) of average daily net assets until November 1, 2015. The contractual agreement may be terminated upon 90 days' notice by a majority of the non-interested directors of the Fund or by a vote of a majority of the outstanding voting securities of the Fund.

[1] The Custom National Index reflects the returns of the S&P® Municipal Bond Index for periods prior to January 1, 2013, and the returns of only those bonds in the S&P® Municipal Bond Index that have maturities greater than 5 years for periods subsequent to January 1, 2013.