For decades, the Beale Street Boy has ably served as the tireless de facto ambassador of the blues, maintaining a punishing touring schedule taking him around the world—at age 88, to boot. As one of the last living links to the wellsprings of that storied genre, a B.B. King show is more than just another concert; it’s a precious glimpse into a disappearing past. The Moore. 7:30 p.m. $52.50 and up. CORBIN REIFF