One of the most formally and politically groundbreaking movies of the early 1970s, Melvin Van Peebles’ 1971 trailblazer Sweet Sweetback’s Baadasssss Song, is now available on Blu-ray from specialty label Vinegar Syndrome, a company doing more to preserve the heritage of America’s outlaw independent films than any other. Sweetback is one of the more historically significant titles in the company’s catalogue, a picture so influential that its innovations are probably less readily apparent to contemporary audiences than they should be — though the propulsive energy and stylistic audacity that drive the movie still set it apart from its many imitators. […]