Just as major chords can be altered to form 7ths, minor chords can be altered to form minor 7ths. Here are the shapes for the root 6 and root5 minor bar chords and minor 7th bar chords. Learn each one from memory and then play example 14.
In minor keys chords I and IV are usually minor. Chord V is sometimes minor and sometimes a 7th, as shown in this 12 bar Blues in the key of A minor. In this example, chord I is root 6, while chords IV and V are root 5. The E7 chord in the final bar is played as an open chord.