Well you can always do the standard blues scale. If you want to add a little "spice" try this:
Key of Ab
Ab, Gb, F, Eb, D, Db, B, Ab, F, Eb, C, B, Bb, Ab, Gb, F, Eb, D, D, Db, B, Ab
Just repeat as needed. If you need the organ to "scream", simply hit Ab Eb D B