There was a docudrama shown years ago on cable about Leadbelly. I think that was the name of it, also. Gordon Parks Jr. had something to do with it.
Also, I've read and heard about a documentary called "Deep Blue" or "Deep Blues," supposedly done by by Dave Stewart (the same guy from the Eurythmics?). I know for sure it was released theatrically several years ago, but don't know that it was ever put out on video (I've been looking for it). I even found something on the web about a soundtrack album on Atlantic, compiled by the late rock journalist Robert Palmer, but can't find that, either.
There was also a documentary called "Good Morning, Blues" that I found at the local library. It might have been an old PBS production.