Keep it simple -- that's the secret behind this unadorned frolic. The music is basic: a chain-gang song with numerous verses (''Shoo Fly Don't Bother Me''), a spiritual (Dr. John's attractive minor-key variation on ''The Saints,'' with a wailing Fathead alto sax solo), and three 12-bar blues. Doc does most of the singing and slips and slides over the piano keys; Fathead, who was Ray Charles' most celebrated bandmate, switches between alto and tenor saxes and flute. The unexpected payoff comes when Blakey, the master drummer, sits down at the piano and sings the pop classic ''For All We Know.'' His voice is a croak, his vibrato wobbles shamelessly, and his last note never quite finds its mark, but he sustains a lazily swinging backbeat groove that's hard to resist.