Years ago, a radio announcer asked, “Who knows what evil lurks in the hearts of men?” then answered, “The Shadow knows.”

Big deal. Evil’s easy. Does the Shadow know who’s playing the Gospel Tent the first Friday at 1:30 p.m.? Or who leads the Fi Yi Yi Mardi Gras Indians? The Shadow doesn’t have a clue. To help you keep track of the small army of musicians playing the blues, jazz—traditional and modern—R&B, funk, Cajun, zydeco, rock, gospel—traditional and...