In response to SlimPickensIII's comment:
It's a tricky thing to define. To my ears, it's defined by the early Mississippi masters. The music they made there acoustically, and the adaptations they made to it with electric instruments up in Chicago. Tempo would be a part of that definition, mostly it's going to be on the slower side. It gets hot down there, you don't want to move to quickly. And you don't want to move too much either, so not so many notes in it either. I'm only half kidding, and yes it all falls apart when you move to Chicago.
I don't want you to confuse what I'm saying with the attitudes of a blues purist (or a blues nazi as they're sometimes called). I'm all for modifying and tweaking this base sound. Doesn't have to be black or white or American or English. In the right hands it's a good thing.
But don't forget about the Texas Blues such as the music of Blind Lemon Jefferson and T-Bone Walker. And John Lee Hooker's unique free form approach coming out of Detroit.