We’ve just strummed through a blues using the E, A, and B7 chords. Perhaps you
find that it doesn’t sound all that bluesy yet. Play it again, but this time use
a seventh chord every time, using these chord shapes.
Seventh chords sound bluesy because—depending on exactly which chords they are—they
include some extra notes called blue notes.
Practice strumming these chord changes until you can play them smoothly before
we go further. Keep the beat even, and try experimenting by choosing to play any
given measure with either a plain chord or the seventh. If you don’t know a plain
B yet, just use B7. Once you get these chord changes automatic, you’ve got the blues.