Don Rico is right- Robert Johnson is practically the creator of the blues, and while his stuff is a bit repetitive, you'll recognize a bunch of his songs, because they've been covered by other artists. He led a strange life, too- rumor has it he sold his soul to the devil at a crossroads to become great at the guitar, but then supposedly died walking around on all fours and slobbering like a dog. It's a shame there's only about 30 songs (I think) by him, and they are all very rough. I think there are only two photos of him.
I would check out Robert Johnson just to hear the roots of the blues. Here's some other good blues musicians (other then those already mentioned), just off the top of my head.
Chuck Berry- He turned blues into rock and roll. Most of his songs follow the same format (kinda like Robert Johnson from above), but it's fun and more faster-paced.
Little Walter- one of the first to "electrify" the harmonica- very cool stuff. I have his CD called "Little Walter: His Best" (And notice he actually spells out the "Little" in his name!). Nice guitar work by his supporting band too. He worked with the Chess record label- check them out, because a lot of big blues musicians worked with Chess.
George Thorogood and the Destroyers- more modern (70s to present), but faithful to the blues. I love his slide guitar work, but his cover of the song "One Bourbon, One Scotch, One Beer" is really awesome.
Led Zeppelin- No bull! A lot of 60s/70s bands covered old blues songs. Zeppelin's blues songs are scattered throughout their records. My personal favorites are "Hats off to Roy Harper" (a sweet song with slide guitar and Robert Plant's vocals treated electronically to waver in volume- another version of this can be found at one of the major Zeppelin websites, but I'm not sure which), Robert Johnson's "Travelling Riverside Blues" (another great slide guitar piece), and "I Can't Quit You Baby" (awesome guitar riffs by Page).
I know that's a lot of. . . variety, but I listened to a lot of blues music before arriving in the world of psychedelia. Once you hear that very simple blues shuffle, you'll recognize it in a lot of music, and you can hear the bluesiness in a lot of JA's music.
I say try out all types of blues music- acoustic, electric, slide, harmonica, etc as well as different locations of music- Chicago, the "Delta," etc. I hope this little list helps you out!