Watch the Compin' online guitar lesson by Jeff McErlain from 50 Blues Guitar Licks You MUST Know
Not sure what to play behind that singer or soloist, well here is an idea. This lick outlines the I7 IV7 and V7 chords in an A blues. I use my fingers on my picking hand on this one for a warmer tone and a piano like separation between the bass notes and chord. Check out all the greats and you will see how much they DON'T use a pick. One of the best lessons I ever learned was to put the pick down.