Hi,
This blues discussion is scary ... and I can only appreciate the time and effort that Dark Elf took in putting it together to show something.
There is one other small detail ... and I would like to add (and suggest DE see it) the DVD on Tom Dowd and the fact that after WW2, he started recording a lot of artists and many of them were black and into the blues and jazz. His comment on the video is that the music studios owned all the music publishing and recording studios and they only released the material that they could sell for their "stars" ... and thus, a sectional group, as most of these "blues" and "blacks" were across the country, would not exactly be known by anyone else, and could easily be taken away and become another song ... which might not have been intentional or malignant ... all you need is a friend from the East Coast come visit you on the West Coast, and he/she hums a song, and then you pick it up and before you know it, you have something else ... that could be said to have come from that other piece.
One last thing ... that is VERY IMPORTANT here ... up to 50 years ago, most of this stuff was "invisible" ... only a handful of folks heard of Johnson this or that, or Sonny this or that ... today, with the media and internet and wiki, it's easy to all of a sudden "know" them all, and putting together a history is not a difficult process, although the details and the geography of it all is way too immense for most studies. But this was not there 50 years ago, and again, a lot of the "black" music was not available as widely as it is now.
This, is also a great lesson on music and its changes and history ... something almost similar also happened with "progressive" music, and led to places like "ProgArchives" being created, and helped maintain a strong connection to the number of artists, although not quite/exactly its history which is in the hands of a few (like DE, or Dean and others) ... and is not put together (as it probably should) to help explain the "progressive" gestation a lot better for yournger folks, that even post in this board.
The only issue might be ... that there are too many differences, and America is like 5 or 6 different countries and the black history in California is not the same as the black history in Alabama ... and it shows even in the music!