Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

BLACKROCK MUNICIPAL BOND FUND, INC.

Prospectus Date

rr_ProspectusDate

Oct. 28, 2014

Supplement [Text Block]

bmbfi_SupplementTextBlock

BLACKROCK MUNICIPAL BOND FUND, INC.
BlackRock National Municipal Fund
(the "Fund")

Supplement dated October 1, 2015
to the Service Shares Summary Prospectus and
Service Shares Prospectus of the Fund,
each dated October 28, 2014

Effective as of October 1, 2015, BlackRock Advisors, LLC has agreed contractually to reduce its management fee for the Fund. Due to certain expense caps, the management fee reduction may not result in a decrease of total annual fund operating expenses after fee waivers and/or reimbursements.

Accordingly, the Fund's Summary Prospectus and Prospectus are amended as follows:

The section entitled "Fund Overview — Key Facts About BlackRock National Municipal Fund — Fees and Expenses of the Fund" in each of the Summary Prospectus and Prospectus is deleted in its 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.

Shareholder Fees (fees paid directly from your investment)	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)	Service Shares
Management Fee	0.43%
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	0.85%

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	$87	$271	$471	$1,049

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.

BlackRock National Municipal Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

bmbfi_SupplementTextBlock

BLACKROCK MUNICIPAL BOND FUND, INC.
BlackRock National Municipal Fund
(the "Fund")

Supplement dated October 1, 2015
to the Service Shares Summary Prospectus and
Service Shares Prospectus of the Fund,
each dated October 28, 2014

Effective as of October 1, 2015, BlackRock Advisors, LLC has agreed contractually to reduce its management fee for the Fund. Due to certain expense caps, the management fee reduction may not result in a decrease of total annual fund operating expenses after fee waivers and/or reimbursements.

Accordingly, the Fund's Summary Prospectus and Prospectus are amended as follows:

The section entitled "Fund Overview — Key Facts About BlackRock National Municipal Fund — Fees and Expenses of the Fund" in each of the Summary Prospectus and Prospectus is deleted in its 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.

Shareholder Fees (fees paid directly from your investment)	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)	Service Shares
Management Fee	0.43%
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	0.85%

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	$87	$271	$471	$1,049

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.

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:

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

0.43%

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

0.85%

1 Year

rr_ExpenseExampleYear01

$ 87

3 Years

rr_ExpenseExampleYear03

271

5 Years

rr_ExpenseExampleYear05

471

10 Years

rr_ExpenseExampleYear10

$ 1,049