Please note: these instructions are only applicable for docx files. If your file extension is .doc, see Converting a doc file to docx file. The doc file extension is an older file type that does not include the accessibility checker functionality.
Microsoft's support site includes instructions on how to Improve accessibility with the Accessibility Checker. You'll want to check the accessibility in your documents before uploading them to your course in Blackboard.
Once you have uploaded your file to Blackboard, you can use Anthology (Blackboard) A11Y to further check and improve your document's accessibility.