But the version of “Harmony” that opened on Monday at the Ethel Barrymore Theater, after a potholed, decades-long trek to Broadway, makes a beeline for the bleakest parts of the tale and then bleakens them further. Sussman’s script, relentlessly focused on historical trauma, takes reasonable dramatic license with the group’s actual history, but only in one direction: darker. And though Warren Carlyle’s production is smart and slick, it traps the tale in a figurative and literal glassy black box (by Beowulf Boritt) from which only pathos escapes.