497

  787 Seventh Avenue New York, NY 10019-6099 Tel: 212 728 8000 Fax: 212 728 8111

June 19, 2018

VIA EDGAR

Securities and Exchange Commission

100 F Street, NE

Washington, DC 20549

Re: BlackRock National Municipal Fund, a series of BlackRock Municipal Bond Fund, Inc.

(File No. 002-57354 and File No. 811-02688)

Ladies and Gentlemen:

On behalf of BlackRock Municipal Bond Fund, Inc. and pursuant to Rule 497(e) under the Securities Act of 1933, as amended, attached for filing are exhibits containing interactive data format risk/return summary information that mirrors the risk/return summary information in a supplement, dated May 30, 2018, to the Prospectus, dated October 27, 2017, for BlackRock National Municipal Fund (the “Fund”). The purpose of the filing is to submit the 497(e) filing dated May 30, 2018 in XBRL for the Fund.

Any questions or comments on this filing should be directed to the undersigned at (212) 728-8955.

Very truly yours,

/s/ Bissie K. Bonner

Bissie K. Bonner

Enclosures

cc: Benjamin Archibald, Esq., BlackRock Advisors, LLC

Elliot J. Gluck, Esq., Willkie Farr & Gallagher LLP

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Document Type	dei_DocumentType	497
Document Period End Date	dei_DocumentPeriodEndDate	Jun. 30, 2017
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	May 30, 2018
Document Effective Date	dei_DocumentEffectiveDate	May 30, 2018
Prospectus Date	rr_ProspectusDate	Oct. 27, 2017

XML

Oct. 27, 2017

BLACKROCK MUNICIPAL BOND FUND, INC.

BlackRock National Municipal Fund

(the “Fund”)

Investor and Institutional Shares

Supplement dated May 30, 2018

to the Summary Prospectus and Prospectus, each dated October 27, 2017, as supplemented to date

Effective May 31, 2018, BlackRock Advisors, LLC (“BlackRock”) has agreed to adjust the caps on total expenses to reduce the net expenses paid by shareholders of the Fund. To achieve these expense caps, BlackRock has agreed to waive and/or reimburse fees and/or expenses if the Fund’s annual fund operating expenses, excluding certain expenses described in the prospectus, exceed a certain limit for the Fund’s Investor A, Investor C and Institutional Shares. Accordingly, effective May 31, 2018, the Fund’s Summary Prospectus and Prospectus are amended as follows:

The section of the Fund’s Summary Prospectus entitled “Key Facts About BlackRock National Municipal Fund — Fees and Expenses of the Fund” and the section of the Fund’s Prospectus entitled “Fund Overview — Key Facts About BlackRock National Municipal Fund — Fees and Expenses of the Fund” are deleted in their entirety and replaced with the following:

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” and the “Intermediary-Defined Sales Charge Waiver Policies” sections on pages 37 and A-1, respectively, of the Fund’s prospectus and in the “Purchase of Shares” section on page II-73 of Part II of the Fund’s Statement of Additional Information.

Shareholder Fees (fees paid directly from your investment)	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)	Investor A Shares			Investor C Shares			Institutional Shares
Management Fee3,4,5		0.39	%		0.39	%		0.39	%
Distribution and/or Service (12b-1) Fees		0.25	%		1.00	%		None
Other Expenses		0.14	%		0.09	%		0.15	%
Interest Expense		0.01%			0.01%			0.01%
Miscellaneous Other Expenses		0.13%			0.08%			0.14%
Acquired Fund Fees and Expenses5		0.01	%		0.01	%		0.01	%
Total Annual Fund Operating Expenses5		0.79	%		1.49	%		0.55	%
Fee Waivers and/or Expense Reimbursements4,6		(0.09	)%		(0.04	)%		(0.10	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements4,6		0.70	%		1.45	%		0.45	%

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 as part of an investment of $250,000 or more.

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

³  The Management Fee has been restated to reflect current fees.

⁴  As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 53, BlackRock has contractually agreed to waive the management fee with respect to any portion of the Fund’s assets estimated to be attributable to investments in other equity and fixed-income mutual funds and exchange-traded funds managed by BlackRock or its affiliates that have a contractual management fee, through October 31, 2019. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested directors of BlackRock Municipal Bond Fund, Inc. (the “Corporation”) or by a vote of a majority of the outstanding voting securities of the Fund.

⁵  The Total Annual Fund Operating Expenses do not correlate to the ratios of expenses to average net assets given in the Fund’s most recent annual report, which do not include the restatement of Management Fees to reflect current fees and Acquired Fund Fees and Expenses.

⁶  As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 53, 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 0.68% (for Investor A Shares), 1.43% (for Investor C Shares) and 0.43% (for Institutional Shares) of average daily net assets through October 31, 2019. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested directors of the Corporation or by a vote of a majority of the outstanding voting securities of the Fund.

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:

	1 Year		3 Years		5 Years		10 Years
Investor A Shares	$	493	$	657	$	835	$	1,349
Investor C Shares	$	248	$	466	$	808	$	1,772
Institutional Shares	$	46	$	166	$	297	$	680

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

	1 Year		3 Years		5 Years		10 Years
Investor C Shares	$	148	$	466	$	808	$	1,772

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 83% of the average value of its portfolio.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

BLACKROCK MUNICIPAL BOND FUND, INC.

Prospectus Date

rr_ProspectusDate

Oct. 27, 2017

Supplement [Text Block]

bmbfi3_SupplementTextBlock

BLACKROCK MUNICIPAL BOND FUND, INC.

BlackRock National Municipal Fund

(the “Fund”)

Investor and Institutional Shares

Supplement dated May 30, 2018

to the Summary Prospectus and Prospectus, each dated October 27, 2017, as supplemented to date

Effective May 31, 2018, BlackRock Advisors, LLC (“BlackRock”) has agreed to adjust the caps on total expenses to reduce the net expenses paid by shareholders of the Fund. To achieve these expense caps, BlackRock has agreed to waive and/or reimburse fees and/or expenses if the Fund’s annual fund operating expenses, excluding certain expenses described in the prospectus, exceed a certain limit for the Fund’s Investor A, Investor C and Institutional Shares. Accordingly, effective May 31, 2018, the Fund’s Summary Prospectus and Prospectus are amended as follows:

The section of the Fund’s Summary Prospectus entitled “Key Facts About BlackRock National Municipal Fund — Fees and Expenses of the Fund” and the section of the Fund’s Prospectus entitled “Fund Overview — Key Facts About BlackRock National Municipal Fund — Fees and Expenses of the Fund” are deleted in their entirety and replaced with the following:

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” and the “Intermediary-Defined Sales Charge Waiver Policies” sections on pages 37 and A-1, respectively, of the Fund’s prospectus and in the “Purchase of Shares” section on page II-73 of Part II of the Fund’s Statement of Additional Information.

Shareholder Fees (fees paid directly from your investment)	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)	Investor A Shares			Investor C Shares			Institutional Shares
Management Fee3,4,5		0.39	%		0.39	%		0.39	%
Distribution and/or Service (12b-1) Fees		0.25	%		1.00	%		None
Other Expenses		0.14	%		0.09	%		0.15	%
Interest Expense		0.01%			0.01%			0.01%
Miscellaneous Other Expenses		0.13%			0.08%			0.14%
Acquired Fund Fees and Expenses5		0.01	%		0.01	%		0.01	%
Total Annual Fund Operating Expenses5		0.79	%		1.49	%		0.55	%
Fee Waivers and/or Expense Reimbursements4,6		(0.09	)%		(0.04	)%		(0.10	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements4,6		0.70	%		1.45	%		0.45	%

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 as part of an investment of $250,000 or more.

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

³  The Management Fee has been restated to reflect current fees.

⁴  As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 53, BlackRock has contractually agreed to waive the management fee with respect to any portion of the Fund’s assets estimated to be attributable to investments in other equity and fixed-income mutual funds and exchange-traded funds managed by BlackRock or its affiliates that have a contractual management fee, through October 31, 2019. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested directors of BlackRock Municipal Bond Fund, Inc. (the “Corporation”) or by a vote of a majority of the outstanding voting securities of the Fund.

⁵  The Total Annual Fund Operating Expenses do not correlate to the ratios of expenses to average net assets given in the Fund’s most recent annual report, which do not include the restatement of Management Fees to reflect current fees and Acquired Fund Fees and Expenses.

⁶  As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 53, 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 0.68% (for Investor A Shares), 1.43% (for Investor C Shares) and 0.43% (for Institutional Shares) of average daily net assets through October 31, 2019. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested directors of the Corporation or by a vote of a majority of the outstanding voting securities of the Fund.

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:

	1 Year		3 Years		5 Years		10 Years
Investor A Shares	$	493	$	657	$	835	$	1,349
Investor C Shares	$	248	$	466	$	808	$	1,772
Institutional Shares	$	46	$	166	$	297	$	680

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

	1 Year		3 Years		5 Years		10 Years
Investor C Shares	$	148	$	466	$	808	$	1,772

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 83% of the average value of its portfolio.

BlackRock National Municipal Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

bmbfi3_SupplementTextBlock

BLACKROCK MUNICIPAL BOND FUND, INC.

BlackRock National Municipal Fund

(the “Fund”)

Investor and Institutional Shares

Supplement dated May 30, 2018

to the Summary Prospectus and Prospectus, each dated October 27, 2017, as supplemented to date

Effective May 31, 2018, BlackRock Advisors, LLC (“BlackRock”) has agreed to adjust the caps on total expenses to reduce the net expenses paid by shareholders of the Fund. To achieve these expense caps, BlackRock has agreed to waive and/or reimburse fees and/or expenses if the Fund’s annual fund operating expenses, excluding certain expenses described in the prospectus, exceed a certain limit for the Fund’s Investor A, Investor C and Institutional Shares. Accordingly, effective May 31, 2018, the Fund’s Summary Prospectus and Prospectus are amended as follows:

The section of the Fund’s Summary Prospectus entitled “Key Facts About BlackRock National Municipal Fund — Fees and Expenses of the Fund” and the section of the Fund’s Prospectus entitled “Fund Overview — Key Facts About BlackRock National Municipal Fund — Fees and Expenses of the Fund” are deleted in their entirety and replaced with the following:

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” and the “Intermediary-Defined Sales Charge Waiver Policies” sections on pages 37 and A-1, respectively, of the Fund’s prospectus and in the “Purchase of Shares” section on page II-73 of Part II of the Fund’s Statement of Additional Information.

Shareholder Fees (fees paid directly from your investment)	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)	Investor A Shares			Investor C Shares			Institutional Shares
Management Fee3,4,5		0.39	%		0.39	%		0.39	%
Distribution and/or Service (12b-1) Fees		0.25	%		1.00	%		None
Other Expenses		0.14	%		0.09	%		0.15	%
Interest Expense		0.01%			0.01%			0.01%
Miscellaneous Other Expenses		0.13%			0.08%			0.14%
Acquired Fund Fees and Expenses5		0.01	%		0.01	%		0.01	%
Total Annual Fund Operating Expenses5		0.79	%		1.49	%		0.55	%
Fee Waivers and/or Expense Reimbursements4,6		(0.09	)%		(0.04	)%		(0.10	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements4,6		0.70	%		1.45	%		0.45	%

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 as part of an investment of $250,000 or more.

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

³  The Management Fee has been restated to reflect current fees.

⁴  As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 53, BlackRock has contractually agreed to waive the management fee with respect to any portion of the Fund’s assets estimated to be attributable to investments in other equity and fixed-income mutual funds and exchange-traded funds managed by BlackRock or its affiliates that have a contractual management fee, through October 31, 2019. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested directors of BlackRock Municipal Bond Fund, Inc. (the “Corporation”) or by a vote of a majority of the outstanding voting securities of the Fund.

⁵  The Total Annual Fund Operating Expenses do not correlate to the ratios of expenses to average net assets given in the Fund’s most recent annual report, which do not include the restatement of Management Fees to reflect current fees and Acquired Fund Fees and Expenses.

⁶  As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 53, 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 0.68% (for Investor A Shares), 1.43% (for Investor C Shares) and 0.43% (for Institutional Shares) of average daily net assets through October 31, 2019. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested directors of the Corporation or by a vote of a majority of the outstanding voting securities of the Fund.

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:

	1 Year		3 Years		5 Years		10 Years
Investor A Shares	$	493	$	657	$	835	$	1,349
Investor C Shares	$	248	$	466	$	808	$	1,772
Institutional Shares	$	46	$	166	$	297	$	680

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

	1 Year		3 Years		5 Years		10 Years
Investor C Shares	$	148	$	466	$	808	$	1,772

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 83% of the average value of its portfolio.

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” and the “Intermediary-Defined Sales Charge Waiver Policies” sections on pages 37 and A-1, respectively, of the Fund’s prospectus and in the “Purchase of Shares” section on page II-73 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

October 31, 2019

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 83% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

83.00%

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 as part 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 $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 Restated to Reflect Current [Text]

rr_ExpensesRestatedToReflectCurrent

The Management Fee has been restated to reflect current fees.

Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text]

rr_ExpensesNotCorrelatedToRatioDueToAcquiredFundFees

The Total Annual Fund Operating Expenses do not correlate to the ratios of expenses to average net assets given in the Fund’s most recent annual report, which do not include the restatement of Management Fees to reflect current fees and 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:

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

0.39%

^[2],[3],[4]

Distribution and/or Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

0.25%

Interest Expense

rr_Component1OtherExpensesOverAssets

0.01%

Miscellaneous Other Expenses

rr_Component2OtherExpensesOverAssets

0.13%

Other Expenses

rr_OtherExpensesOverAssets

0.14%

Acquired Fund Fees and Expenses

rr_AcquiredFundFeesAndExpensesOverAssets

0.01%

^[4]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

0.79%

^[4]

Fee Waivers and/or Expense Reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.09%)

^[2],[5]

Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements

rr_NetExpensesOverAssets

0.70%

^[2],[5]

1 Year

rr_ExpenseExampleYear01

$ 493

3 Years

rr_ExpenseExampleYear03

657

5 Years

rr_ExpenseExampleYear05

835

10 Years

rr_ExpenseExampleYear10

$ 1,349

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%

^[6]

Management Fee

rr_ManagementFeesOverAssets

0.39%

^[2],[3],[4]

Distribution and/or Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

1.00%

Interest Expense

rr_Component1OtherExpensesOverAssets

0.01%

Miscellaneous Other Expenses

rr_Component2OtherExpensesOverAssets

0.08%

Other Expenses

rr_OtherExpensesOverAssets

0.09%

Acquired Fund Fees and Expenses

rr_AcquiredFundFeesAndExpensesOverAssets

0.01%

^[4]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.49%

^[4]

Fee Waivers and/or Expense Reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.04%)

^[2],[5]

Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements

rr_NetExpensesOverAssets

1.45%

^[2],[5]

1 Year

rr_ExpenseExampleYear01

$ 248

3 Years

rr_ExpenseExampleYear03

466

5 Years

rr_ExpenseExampleYear05

808

10 Years

rr_ExpenseExampleYear10

1,772

1 Year

rr_ExpenseExampleNoRedemptionYear01

148

3 Years

rr_ExpenseExampleNoRedemptionYear03

466

5 Years

rr_ExpenseExampleNoRedemptionYear05

808

10 Years

rr_ExpenseExampleNoRedemptionYear10

$ 1,772

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

0.39%

^[2],[3],[4]

Distribution and/or Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

none

Interest Expense

rr_Component1OtherExpensesOverAssets

0.01%

Miscellaneous Other Expenses

rr_Component2OtherExpensesOverAssets

0.14%

Other Expenses

rr_OtherExpensesOverAssets

0.15%

Acquired Fund Fees and Expenses

rr_AcquiredFundFeesAndExpensesOverAssets

0.01%

^[4]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

0.55%

^[4]

Fee Waivers and/or Expense Reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.10%)

^[2],[5]

Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements

rr_NetExpensesOverAssets

0.45%

^[2],[5]

1 Year

rr_ExpenseExampleYear01

$ 46

3 Years

rr_ExpenseExampleYear03

166

5 Years

rr_ExpenseExampleYear05

297

10 Years

rr_ExpenseExampleYear10

$ 680

[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 as part of an investment of $250,000 or more.

[2] As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 53, BlackRock has contractually agreed to waive the management fee with respect to any portion of the Fund’s assets estimated to be attributable to investments in other equity and fixed-income mutual funds and exchange-traded funds managed by BlackRock or its affiliates that have a contractual management fee, through October 31, 2019. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested directors of BlackRock Municipal Bond Fund, Inc. (the “Corporation”) or by a vote of a majority of the outstanding voting securities of the Fund.

[3] The Management Fee has been restated to reflect current fees.

[4] The Total Annual Fund Operating Expenses do not correlate to the ratios of expenses to average net assets given in the Fund’s most recent annual report, which do not include the restatement of Management Fees to reflect current fees and Acquired Fund Fees and Expenses.

[5] As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 53, 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 0.68% (for Investor A Shares), 1.43% (for Investor C Shares) and 0.43% (for Institutional Shares) of average daily net assets through October 31, 2019. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested directors of the Corporation or by a vote of a majority of the outstanding voting securities of the Fund.

[6] 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. 27, 2017
Document Creation Date	dei_DocumentCreationDate	May 30, 2018