Here is some good basic Blues guitar chord charts. Which are all the sevenths.
Below this chart is all the other basic open chords for the Blues.
The classic Key of C (C7, F7 and G7) One of Stevie Ray Vaughan’s favorite keys to play in.
With one of his greatest songs Texas Flood.
In one of our older posts (Blues Guitar Lesson from Stevie Ray Vaughan.)
but in the key of E is his instrumental Rude Mood.
Here is some great open chords, which does give you the ability and imagination to play open strings. Musi-Source will post more info on blues guitar chords, leads, and scales to practice with.