I'm kind of shocked there is no mention of Taj Mahal up on here, he has some pretty good stuff. I find myself listening to him and B.B. King most of the time. There is traditional blues, and then there is some of the music that people don't usually think of as blues, such as Ray Charles.
Keb' Mo', Johnny Lang, Odetta, Bessie Smith, Paul Michael, Safhire, Bonnie Raitt, Bob Dylan, John Lee Hooker, Joan Osborn, just to name a few.
The more you progress in time the more you see rock and blues mixing. You can define the two by the distinct beats (suck as the 12 step beat). More so, blues, folk, rock, soul, country (old non-pop) and jazz are all linked together, it's hard to tell what is and isn't. Such as the the early Beatles recordings are arguably blues songs, as well as with other rock bands
Blues is what you want it to be, it's music (to me at least) that has a good beat, tells a story, and above all carries the most emotion from the soul out to the listener.