A few of my sisters recently asked me the difference between southern soul and blues. I didn't realize how hard the question was until I tried to come up with a definition that was clear and concise. When I think of blues, I think of traditional music from artists such as Muddy Waters, Howlin' Wolf, and others. Southern soul takes me to the music of Johnnie Taylor, Tyrone Davis, and the new artists on the scene. Southern soul started in the south and, to me, is a by product of the blues. Daddy B. Nice, one of the best southern soul researchers/writers I've seen, has a great description - see http://www.southernsoulrnb.com/southern_soul_and_blues.cfm. You may also want to take a look at the Southern Soul Evolution at http://www.getbluesinfo.com/.
I'm still working on a solid definition but these two say it way better than me.