Actually, Lighter, I posted that quote from Lucy McKim Garrison because it indicated that she and her associates were very aware of the bent and altered notes--a couple places in her notes in "Slave Songs" she gives the caveat that they were not able to accurately represent what was sung--at any rate, what she heard was probably sounded a bit like this:
Georgia Sea Island Singers Listen and tell me if you hear any blue notes.

Note:The Georgia Sea Island Singers were formed in the early 20th century and performed, preserved, and propagated their music, which reflects the Gullah traditions, which are generally considered to incorporate a lot of fairly undiluted African musical elements.