Watch the Emily's List online guitar lesson by David Hamburger from 30 Blues Intros, Turnarounds, & Endings You MUST Know
This ending is a slow-blues variation on a series of voicings I learned from Duke Robillard in a swing context. Check out the descending line on the second string, from F# to F to E to Eb. The theory cats call that a guide tone line, and it gives the chord progression a great sense of direction. It's also a useful thing to incorporate into your single-note improvising. Just sayin'.