“Boyz N the Hood” rests in American movie history like a boulder in a riverbed, altering the direction of the stream. After its release in the summer of 1991, everything looked different, including its precursors. “Mean Streets,” “Rebel Without a Cause,” the Blaxploitation spectacles of the 1970s, the socially conscious crime dramas of the 1930s, classic westerns and samurai epics — somehow John Singleton, a very recent graduate of the University of Southern California film school, synthesized all of those models even as he came up with something bracingly, thrillingly and frighteningly new.

“Boyz” made him the youngest person — and the first African-American — nominated for a best directing Academy Award. In the annals of cinema, there aren’t many first features to match it for ambition and impact (“Citizen Kane”? “Breathless”?), and its influence on what came after is hard to overstate. Singleton, who died Monday at 51, filled his characters’ lives with warmth and humor even as they were constantly menaced, and often destroyed, by violence. He infused familiar coming-of-age and gangster-movie tropes with a rare authenticity. This wasn’t just a matter of his intimate knowledge of the setting known then as South-Central Los Angeles, but also of his brave, even brazen confidence in himself and his audience.