On April 26, three years after Bayou Maharajah began making the film festival rounds, Lily Keber’s documentary about New Orleans piano genius James Booker will make its DVD and online streaming debuts.

Booker, a classically trained virtuoso who dazzled his fellow New Orleans musicians, led a colorful, troubled life ripe for legend status. In 1983, he died at 43 in the Charity Hospital wait...