the chess blues box is pretty nice
Lots of blues labels have some good samplers: Evidence has a couple or three of them, focusing on guitar, harmonica, and, I think, keyboards. Also, Blues Revue mag. has been putting out some samplers. They contain a pretty ecclectic mix, some of which is not to my taste, but, it's quite a selection; and the price is right (Free!). Finally, I have an Atlantic blues sampler that has Buddy Guy, Luther Allison, Howlin Wolf, Muddy Waters, etc.--and it has been one of my favorite blues CDs for a long time. I think that it's called "The Great Chicago Guitarists" or something like that. It's a Good Un!