If you're into gospel and blues, you'll enjoy this tribute to Thomas A. Dorsey enormously. At a time when black music was rigidly categorized as "sacred" or "profane," Dorsey (who died in January at 93) fused a bluesman's bumptious earthiness with a churchman's rapturous religiosity to create a distinctive form that came to be known as gospel; this musical biography by Jackie Taylor and Jimmy Tillman joyfully reflects its hero's pleasure in the worldly and spiritual elements of song. Precious Lord traces Dorsey's career from the 1920s, when under the name Georgia Tom he made his reputation as pianist for Ma Rainey, through the 30s and 40s, when he turned exclusively to church music as composer, conductor, and advocate. (Like Bach, Dorsey's roles as writer and teacher were inextricably linked; he was a religious musician who was as committed to religion as to music.) The score's highlights range from the infectiously funky "It's Tight Like That," the 1928 hit Dorsey wrote with jazz guitarist Tampa Red, to such soaring, soulful hymns as "Peace in the Valley" and the title tune; viewers are invited to sing along when they know the songs, as many of them will. Precious Lord's one major failing is that it never probes the emotional conflicts Dorsey must have felt as he wrestled with the question of whether to stay in the blues market or to commit himself completely to church music; that failing undercuts the subject of faith at the show's heart, for faith is never attained without a struggle. But its simple, pageantlike style (including sequences of choral speaking, confessional monologue, ritual dance, and plenty of fabulous solo and harmony singing) proves surprisingly entertaining, thanks to the disarming ingenuousness of the cast--headed by Robert W. Barnett as Dorsey, Alice Clark Brown as his wife Nettie, Suzan Chatims as Mahalia Jackson, and wryly comic Cynthia Jackson as Rainey and to the irresistible force and beauty of the score. Black Ensemble, 4520 N. Beacon, 769-4451. Through March 21: Fridays-Saturdays, 8 PM; Sundays, 4 PM. $15.