<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019be60e-4556-74c9-87a0-d3514ca771a0,g:b8d40475-3b63-4a6e-b45a-08962a9a5489-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://xbrl.sec.gov/cef/role/SecurityOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/cef/2025/cef-2025.xsd#SecurityOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/cef/role/SecurityOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_bbdc_August2020NPAMember_019be60e-4556-7f4b-9e4d-0439317adec8" xlink:href="bbdc-20260309.xsd#bbdc_August2020NPAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain" xlink:to="loc_bbdc_August2020NPAMember_019be60e-4556-7f4b-9e4d-0439317adec8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_bbdc_SeriesBSeniorUnsecuredNotesDueNovember2025Member_019be60e-4557-7ea3-87eb-27cf7772af66" xlink:href="bbdc-20260309.xsd#bbdc_SeriesBSeniorUnsecuredNotesDueNovember2025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain" xlink:to="loc_bbdc_SeriesBSeniorUnsecuredNotesDueNovember2025Member_019be60e-4557-7ea3-87eb-27cf7772af66" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_bbdc_February2020NotePurchaseAgreementMember_019be60e-4556-7af0-887c-94fc42788298" xlink:href="bbdc-20260309.xsd#bbdc_February2020NotePurchaseAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain" xlink:to="loc_bbdc_February2020NotePurchaseAgreementMember_019be60e-4556-7af0-887c-94fc42788298" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_bbdc_November2026NotesMember_019be60e-4557-7a8b-9f86-e1489d1eae8e" xlink:href="bbdc-20260309.xsd#bbdc_November2026NotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain" xlink:to="loc_bbdc_November2026NotesMember_019be60e-4557-7a8b-9f86-e1489d1eae8e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_bbdc_February2029NotesMember_019be60e-4556-7ef8-a4aa-a0aa693586a5" xlink:href="bbdc-20260309.xsd#bbdc_February2029NotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain" xlink:to="loc_bbdc_February2029NotesMember_019be60e-4556-7ef8-a4aa-a0aa693586a5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_bbdc_September2028NotesMember_019c77a0-5015-7cfe-b39d-ae1784d6d939" xlink:href="bbdc-20260309.xsd#bbdc_September2028NotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain" xlink:to="loc_bbdc_September2028NotesMember_019c77a0-5015-7cfe-b39d-ae1784d6d939" xlink:type="arc" order="6"/>
  </link:definitionLink>
</link:linkbase>
