I'm a big fan of the Mississippi hill country blues style. RL Burnside, Juke Joint Duo, Junior Kimbrough, Charles Caldwell, T-Model Ford... go to Fat Possum Records' website and go through their catalog... almost every album has 1-3 songs you can download for free. Not all of FP's catalog is blues, but a lot is.
On a related note, rent the movie "You See Me Laughin'" and that will help you with this genre, too. Awesome music played by incredible people, real blues.