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]

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BLACKROCK MUNICIPAL BOND FUND, INC.

BlackRock National Municipal Fund

(the “Fund”)

Service 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 Service 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 Service Shares of the Fund.

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)	Service Shares
Management Fee1,2,3		0.39	%
Distribution and/or Service (12b-1) Fees		0.25	%
Other Expenses		0.09	%
Interest Expense		0.01%
Miscellaneous Other Expenses		0.08%
Acquired Fund Fees and Expenses3		0.01	%
Total Annual Fund Operating Expenses3		0.74	%
Fee Waivers and/or Expense Reimbursements2,4		(0.04	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements2,4		0.70	%

¹  The Management Fee has been restated to reflect current fees.

²  As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 26, BlackRock Advisors, LLC (“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 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.

⁴  As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 26, 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% 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
Service Shares	$	72	$	232	$	406	$	910

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]

bmbfi_SupplementTextBlock

BLACKROCK MUNICIPAL BOND FUND, INC.

BlackRock National Municipal Fund

(the “Fund”)

Service 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 Service 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 Service Shares of the Fund.

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)	Service Shares
Management Fee1,2,3		0.39	%
Distribution and/or Service (12b-1) Fees		0.25	%
Other Expenses		0.09	%
Interest Expense		0.01%
Miscellaneous Other Expenses		0.08%
Acquired Fund Fees and Expenses3		0.01	%
Total Annual Fund Operating Expenses3		0.74	%
Fee Waivers and/or Expense Reimbursements2,4		(0.04	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements2,4		0.70	%

¹  The Management Fee has been restated to reflect current fees.

²  As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 26, BlackRock Advisors, LLC (“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 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.

⁴  As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 26, 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% 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
Service Shares	$	72	$	232	$	406	$	910

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]

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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.

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 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:

BlackRock National Municipal Fund | Service Shares

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

0.39%

^[1],[2],[3]

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

rr_DistributionAndService12b1FeesOverAssets

0.25%

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%

^[3]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

0.74%

^[3]

Fee Waivers and/or Expense Reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.04%)

^[1],[4]

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

rr_NetExpensesOverAssets

0.70%

^[1],[4]

1 Year

rr_ExpenseExampleYear01

$ 72

3 Years

rr_ExpenseExampleYear03

232

5 Years

rr_ExpenseExampleYear05

406

10 Years

rr_ExpenseExampleYear10

$ 910

[1] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 26, BlackRock Advisors, LLC (“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.

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

[3] 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.

[4] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 26, 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% 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.