Here’s a great lesson on blues guitar chords from Marty Schwartz.
The three main chords Marty talks about in this lesson are E7, A7, and B7. These are great sounding open chords, and you have tons of room to improvise over top of them with cool blues riffs like Marty demonstrates in this video.
The other great thing about these particular blues guitar chords is the fact that you can easily modify them, usually by adding your pinky finger to one of the other strings. Again, Marty teaches this in the lesson below.
My favorite riff out of this video is around 3:40 where he does the Stevie Ray Vaughn thing, hitting the G on the third fret as riff or passing note. Man that sounds sweet!