--seed
As far as I know, the familiar 12-bar blues, along with other forms, appeared around the turn of the century. It's a mix of so many different influences though, it's hard to be sure. Like Barry, I recommend Alan Lomax's, "The Land Where The Blues Began". As Barry says, a read. I often refer back to it to get some insight into certain songs and artists. Highly recommended if you're interested in the roots of blues.
Dave T