My one experience with Mac Rebennack/Dr. John was in 1981, on a gig I was on at Tramp's with Big Joe Turner (and subsequently was fired from by Doc Pomus----another story for another day). Dr. John came in during our one rehearsal to pay his respects to the old man. I was on the bandstand and he came up, sat at the piano, started a slow blues---and didn't kick me off.
I don't know about the gris gris stuff, but the man could definitely play the blues.