A stageful of characters you don't often encounter in Broadway houses populates Choir Boy, Tarell Alvin McCraney's two-thirds-drama, one-third-concert, a Manhattan Theatre Club production at the Samuel J. Friedman. (It previously premiered at MTC's Studio Stage II.) That's a welcome thing; surely these African-American teenage boys, struggling with studies, assimilation, and, above all, sexuality, deserve a hearing. And there's plenty of inherent drama in being black and gay; witness the works of Donja R. Love, whose Sugar in Our Wounds and Fireflies got people talking last year. Unlike those, Choir Boy is utterly up-to-the-minute; there's even a Kanye joke, a good one. But like them, it leaves a thread or two hanging. . . .