The midnight fog slips off the Mississippi, moving slowly across the west bank levee like a glimmering shroud. A tanker navigating the treacherous current at New Orleans hugs the Algiers Point embankment. From outside of the Old Point Bar the tanker looks like it’s running aground but it slides past with a mournful foghorn blast, the lingering sound merging with the deep-chested roar of a Hammond B-3. It’s Monday night and John Gros is laying down the organ lines for Papa Grows Funk, the free...