Watch the Stitches online guitar lesson by Ton van Bergeijk from Licks to Riffs Guidebook: Blues
This riff is from a tune called "Stitches" by Al Cooper & His Savoy Sultans - 1939. This orchestra was just about riffs to support its soloists. I find them a constant source of inspiration. The original is over rhythm chords, but it's just as effective over blues chords.