A seventh chord is simply a major chord (notes 1,3,5) with a flatted 7 added. The seventh chord is useful in blues because it adds that "bluesy" sound to the song. Usually, in the 1-4-5 progression of most songs, the 5, which is the "dominant" chord, is best expressed as a 7th chord to get the blues sound.
We played a 12-bar blues by alternating 1 measure of major chords with 1 measure of 7th chords in the blues progression. Sounded pretty cool. Try it!
