The documentary 20 Feet From Stardom, which explores the world of rock ‘n’ roll’s backup singers, opens to the soundtrack of Lou Reed’s “Walk on the Wild Side.” Reed sings half the refrain — “And the colored girls go, doo do doo do doo” — until a chorus of backup singers pick up the “Do doo” line. At first these women sound far away, but as the chorus progresses, their voices get louder, less produced and polished, more real and intimate. Director Morgan Neville tells Fresh Air’s Terry Gross that he opened with the song because “that’s kind of the iconic vision we all have of the backup singers — these three African-American women in the black dresses on the side of the stage, belting it out.” The film chronicles the experience of bringing black voices into different kinds of rock and pop — a revolution that merged church music with rock ‘n’ roll and changed the sound of the latter forever. To tell that story, the film profiles a number of women whose voices you know but whose names you may not: Lisa Fischer, Darlene Love, Judith Hill, Tata Vega and Merry Clayton. It’s Clayton who sang “Rape, murder” on the Rolling Stones song “Gimme Shelter” as if her life depended on it. In some ways, it did. “Everybody wanted me to sing with them,” Clayton tells Gross. “Everybody wanted me to be on their sessions. … I became queen diva of the background rock world.” That’s no exaggeration. Clayton sang with Ray Charles as one of his Raelettes, as well as with Neil Young, Carole King, Elvis, and The Supremes, to list just a few of her credits. MORE