497

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

October 9, 2015

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 October 1, 2015, to the Prospectus dated October 28, 2014, for BlackRock National Municipal Fund (the “Fund”). The purpose of the filing is to submit the 497(e) filing dated October 1, 2015 in XBRL for the Fund.

Any questions or comments on this filing should be directed to the undersigned at 212-728-8037.

Very truly yours,

/s/ Stacey Ruiz

Stacey Ruiz

Enclosures

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

XML

Oct. 28, 2014

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

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

Effective as of October 1, 2015, BlackRock Advisors, LLC (“BlackRock”) 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 BlackRock Shares of the Fund.

Shareholder Fees (fees paid directly from your investment)	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)	BlackRock Shares
Management Fee	0.43%
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	0.56%
Fee Waivers and/or Expense Reimbursements1	(0.04)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements1	0.52%

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.

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
BlackRock Shares	$53	$175	$309	$698

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.

XML

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]

bmbfi3_SupplementTextBlock

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

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

Effective as of October 1, 2015, BlackRock Advisors, LLC (“BlackRock”) 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 BlackRock Shares of the Fund.

Shareholder Fees (fees paid directly from your investment)	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)	BlackRock Shares
Management Fee	0.43%
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	0.56%
Fee Waivers and/or Expense Reimbursements1	(0.04)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements1	0.52%

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.

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
BlackRock Shares	$53	$175	$309	$698

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]

bmbfi3_SupplementTextBlock

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

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

Effective as of October 1, 2015, BlackRock Advisors, LLC (“BlackRock”) 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 BlackRock Shares of the Fund.

Shareholder Fees (fees paid directly from your investment)	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)	BlackRock Shares
Management Fee	0.43%
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	0.56%
Fee Waivers and/or Expense Reimbursements1	(0.04)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements1	0.52%

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.

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
BlackRock Shares	$53	$175	$309	$698

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 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:

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

0.43%

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

0.56%

Fee Waivers and/or Expense Reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.04%)

^[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

175

5 Years

rr_ExpenseExampleYear05

309

10 Years

rr_ExpenseExampleYear10

$ 698

[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.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Document Type	dei_DocumentType	Other
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. 01, 2015
Document Effective Date	dei_DocumentEffectiveDate	Oct. 01, 2015
Prospectus Date	rr_ProspectusDate	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
Document Creation Date	dei_DocumentCreationDate	Oct. 01, 2015