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Label Element Value
Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName BLACKROCK MUNICIPAL BOND FUND, INC.
Prospectus Date rr_ProspectusDate Oct. 28, 2016
Supplement [Text Block] bmbfi3_SupplementTextBlock

BLACKROCK MUNICIPAL BOND FUND, INC.

BlackRock National Municipal Fund

Investor and Institutional Shares

(the “Fund”)

Supplement dated June 9, 2017 to the Prospectus dated October 28, 2016, as amended and supplemented to date

Effective as of June 12, 2017, BlackRock Advisors, LLC (“BlackRock”) has agreed to reduce the Fund’s contractual management fee rate. Accordingly, the Fund’s Prospectus is amended as follows:

The section of the Fund’s Prospectus entitled “Fund Overview — Key Facts About BlackRock National Municipal Fund — Fees and Expenses of the Fund” 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 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 36 and the “Intermediary-Defined Sales Charge Waiver Policies” section of the Fund’s prospectus and in the “Purchase of Shares” section on page II-72 of Part II of the Fund’s Statement of Additional Information.

 

Shareholder Fees
(fees paid directly from your investment)		     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)		     Investor
A
Shares		      Investor
B
Shares		      Investor
C
Shares		      Institutional
Shares		 
Management Fee4, 5      0.39%       0.39%       0.39%       0.39% 
Distribution and/or Service (12b-1) Fees      0.25%       0.75%       1.00%       None 
Other Expenses
   0.16%    0.20%    0.10%    0.16% 
Interest Expense
  0.01%    0.01%    0.01%    0.01%  
Miscellaneous Other Expenses
  0.15%       0.19%       0.09%       0.15%     
Total Annual Fund Operating Expenses6      0.80%       1.34%       1.49%       0.55% 
Fee Waivers and/or Expense Reimbursements5, 7      (0.10)%       (0.14)%       (0.04)%       (0.10)% 
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements5, 7      0.70%       1.20%       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 of an investment of $250,000 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.
4 
The Management Fee has been restated to reflect current fees.
5 
As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 51, 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 through October 31, 2018. 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.
6 
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 expense.
7 
As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 51, 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, 1.19% for Investor B Shares, 1.44% for Investor C Shares and 0.44% for Institutional Shares of average daily net assets through October 31, 2018. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested directors 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    $758    $937    $1,454
Investor B Shares
  $522    $711    $921    $1,600
Investor C Shares
  $248    $467    $809    $1,776
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 B Shares
  $122  $411  $721  $1,600
Investor C Shares
  $148  $467  $809  $1,776

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

Investor and Institutional Shares

(the “Fund”)

Supplement dated June 9, 2017 to the Prospectus dated October 28, 2016, as amended and supplemented to date

Effective as of June 12, 2017, BlackRock Advisors, LLC (“BlackRock”) has agreed to reduce the Fund’s contractual management fee rate. Accordingly, the Fund’s Prospectus is amended as follows:

The section of the Fund’s Prospectus entitled “Fund Overview — Key Facts About BlackRock National Municipal Fund — Fees and Expenses of the Fund” 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 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 36 and the “Intermediary-Defined Sales Charge Waiver Policies” section of the Fund’s prospectus and in the “Purchase of Shares” section on page II-72 of Part II of the Fund’s Statement of Additional Information.

 

Shareholder Fees
(fees paid directly from your investment)		     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)		     Investor
A
Shares		      Investor
B
Shares		      Investor
C
Shares		      Institutional
Shares		 
Management Fee4, 5      0.39%       0.39%       0.39%       0.39% 
Distribution and/or Service (12b-1) Fees      0.25%       0.75%       1.00%       None 
Other Expenses
   0.16%    0.20%    0.10%    0.16% 
Interest Expense
  0.01%    0.01%    0.01%    0.01%  
Miscellaneous Other Expenses
  0.15%       0.19%       0.09%       0.15%     
Total Annual Fund Operating Expenses6      0.80%       1.34%       1.49%       0.55% 
Fee Waivers and/or Expense Reimbursements5, 7      (0.10)%       (0.14)%       (0.04)%       (0.10)% 
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements5, 7      0.70%       1.20%       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 of an investment of $250,000 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.
4 
The Management Fee has been restated to reflect current fees.
5 
As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 51, 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 through October 31, 2018. 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.
6 
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 expense.
7 
As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 51, 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, 1.19% for Investor B Shares, 1.44% for Investor C Shares and 0.44% for Institutional Shares of average daily net assets through October 31, 2018. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested directors 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    $758    $937    $1,454
Investor B Shares
  $522    $711    $921    $1,600
Investor C Shares
  $248    $467    $809    $1,776
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 B Shares
  $122  $411  $721  $1,600
Investor C Shares
  $148  $467  $809  $1,776

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” section on page 36 and the “Intermediary-Defined Sales Charge Waiver Policies” section of the Fund’s prospectus and in the “Purchase of Shares” section on page II-72 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, 2018
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 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.
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]
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Interest Expense rr_Component1OtherExpensesOverAssets 0.01%
Miscellaneous Other Expenses rr_Component2OtherExpensesOverAssets 0.15%
Other Expenses rr_OtherExpensesOverAssets 0.16%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.80% [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 0.70% [2],[5]
1 Year rr_ExpenseExampleYear01 $ 493
3 Years rr_ExpenseExampleYear03 758
5 Years rr_ExpenseExampleYear05 937
10 Years rr_ExpenseExampleYear10 $ 1,454
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% [6]
Management Fee rr_ManagementFeesOverAssets 0.39% [2],[3]
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.75%
Interest Expense rr_Component1OtherExpensesOverAssets 0.01%
Miscellaneous Other Expenses rr_Component2OtherExpensesOverAssets 0.19%
Other Expenses rr_OtherExpensesOverAssets 0.20%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.34% [4]
Fee Waivers and/or Expense Reimbursements rr_FeeWaiverOrReimbursementOverAssets (0.14%) [2],[5]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements rr_NetExpensesOverAssets 1.20% [2],[5]
1 Year rr_ExpenseExampleYear01 $ 522
3 Years rr_ExpenseExampleYear03 711
5 Years rr_ExpenseExampleYear05 921
10 Years rr_ExpenseExampleYear10 1,600
1 Year rr_ExpenseExampleNoRedemptionYear01 122
3 Years rr_ExpenseExampleNoRedemptionYear03 411
5 Years rr_ExpenseExampleNoRedemptionYear05 721
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 1,600
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% [7]
Management Fee rr_ManagementFeesOverAssets 0.39% [2],[3]
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 1.00%
Interest Expense rr_Component1OtherExpensesOverAssets 0.01%
Miscellaneous Other Expenses rr_Component2OtherExpensesOverAssets 0.09%
Other Expenses rr_OtherExpensesOverAssets 0.10%
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 467
5 Years rr_ExpenseExampleYear05 809
10 Years rr_ExpenseExampleYear10 1,776
1 Year rr_ExpenseExampleNoRedemptionYear01 148
3 Years rr_ExpenseExampleNoRedemptionYear03 467
5 Years rr_ExpenseExampleNoRedemptionYear05 809
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 1,776
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]
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Interest Expense rr_Component1OtherExpensesOverAssets 0.01%
Miscellaneous Other Expenses rr_Component2OtherExpensesOverAssets 0.15%
Other Expenses rr_OtherExpensesOverAssets 0.16%
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 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 51, 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 through October 31, 2018. 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 expense.
[5] As described in the "Management of the Funds" section of the Fund's prospectus beginning on page 51, 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, 1.19% for Investor B Shares, 1.44% for Investor C Shares and 0.44% for Institutional Shares of average daily net assets through October 31, 2018. The contractual agreement may be terminated upon 90 days' notice by a majority of the non-interested directors the Corporation or by a vote of a majority of the outstanding voting securities of the Fund.
[6] 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.)
[7] There is no CDSC on Investor C Shares after one year.