This is supposed to be set in 1950 in Alabama (where it was filmed), but the true location is some Never-Never Land in John Sayles’s imagination, sparked by research, a sharp ear for dialogue, and diverse fancies about the birth of rock ‘n’ roll. Yet as in the 1943 musical Stormy Weather, the wonderful cast, mainly black, carries it all with ease, even sailing past occasional false moments, such as a tacky flashback toward the end. Danny Glover, as hard-rock reliable as Spencer Tracy in his prime, plays onetime pianist Tyrone Pine Top Purvis, trying to save his title juke joint from economic disaster by pretending that a young drifter with a guitar (Gary Clark Jr.) is blues star Guitar Sam. He juggles and somehow resolves diverse problems with competition, electricity, cash, his wife, his daughter, and the local sheriff (Stacy Keach), spearheading an overall progress toward communal joy that for me yields the most enjoyable Sayles movie since 1984’s The Brother From Another Planet. PG-13, 123 min. (JR)