Although Blues songs most commonly use 7th chords, there are also many which are played in minor keys. This means that the chords used for the accompaniment are minor chords. Here are three open position minor chords. The chord symbol for the minor chord is a small "m" placed after the letter name.
Here is an exercise to help you become comfortable changing between these chords.
Now try this 12 Bar Blues in the key of A minor. For more about minor keys in general, see Progressive Complete Learn to Play Rhythm Guitar Manual.