NEW YORK (AP) — A long unseen Harlem musical festival with performances by Nina Simone, Stevie Wonder and Sly and the Family Stone has finally seen the light of day. “Summer of Soul (…or When the Revolution Could Not Be Televised),” a concert-film time-capsule of a historic festival, premiered Thursday night on the opening day of the Sundance Film Festival. The film uses 40 hours of mostly never-seen footage from an event known as “Black Woodstock” that never spawned the same amount of attention that another festival in 1969, Woodstock, received. The material had sat in a TV producer’s basement for decades. Questlove, the Roots drummer, turned it into his directorial debut.