So many great points, Chas. I am not any kind of expert musicologist, but I believe blues can be traced back to African slaves in the United States. Singing was a way for them express themselves, connect, and suffer together as a community.

The closest thing that can be found to the slaves songs are field recordings made by folklorist/musicologists in from the 1930's in Southern jails.

Here's a recording of "work" songs from a Mississippi prison during that time period. One can assume that most of the prisoners in this recording are sons and grandsons of slaves, so they may have actually learned these songs from former slaves.