Watch the Jazzy and Slow! online guitar lesson by Brad Carlton from Chord Studies: Slow Blues Progressions Vol. 1
This 12 bar slow blues example is in the key of B flat. You will notice how the same melodic motif occurs on the first string between the I7 and the IV7 chords. There are lots of extended chords here utilizing, altered nines, sharp fives, and some slash chords as well. Dig into this bag next time you want to jazz up a slow blues.