At the risk of a little thread creep, I thought I would respond to pdq's comment earlier in this thread:
"I am not a musicologist, just a fan, but it seems that the chronology of the Blues is:
Minstrel > Ragtime > Trad. Jazz > Blues "
I wonder about this myself. Certainly of all these genres, blues was the last to come to the attention of the wider world, especially in terms of recorded music. Country blues was not really being recorded until well after many of the early jazz masters made it into the recording studio.
But blues music was being sung and played BEFORE Jazz was born, and in fact was one of the primary influences of Jazz music.
Traditional Jazz was the result of an incredible mixing of music, including rural country blues, that happened in cities like New Orleans and Memphis.
When African American music was first being recorded, country blues was seen as rural and "backwards," and often shunned in favour of more urban, sophisticated sounds of Traditional Jazz. This ironically meant that even though it is historically an OLDER form of music, in recorded form it is YOUNGER than Jazz.