As an obsessive SNL watching kid, I first noticed them- somewhere between the lead singer and the band were the backup singers. Almost always, there were three, in identical black dresses. One had an afro, and they stood to the side- they obviously weren't the lead singers, they didn't play instruments, and they certainly weren't in the band.
Sometimes there to add a campy Motown vibe a la Bowie's Young Americans, or perhaps a sense of gravitas/pain- ie, the woman howling as Mick Jagger sings, "Raaaape! Murderrrr!" in Gimme Shelter or whoever was wailing during Dark Side of the Moon, the backup singers were there to add "soul" to the white rock band, or maybe to express the sort of pain even those dudes from Pink Floyd could not. Lou Reed (sort of) acknowledged it when he sang, "And the colored girls say…." before his back-up singers launched into their iconic "Dooo da-doo" chorus in Walk On the Wild Side.
Many rockers who employed those backup singers have a long history of co-opting music traditionally performed and created by black people. No, not Elvis, but all the rock gods who have based their music upon the traditional sound and structure of blues. Mick Jagger adopted an exaggerated faux- Southern accent for The Rolling Stones and sang about honky tonk and the blues despite his British, white, middle-class background. Led Zeppelin and Eric Clapton are a few more who have based their whole careers upon the blues. When Rod Stewart sang Old Man River on a Jeff Beck album, compete with lyrics complaining of hard labor on the Mississippi and struggles under "the white boss", it seems like it was more than okay. It was, and still is, considered cool for these people to adopt what was essentially a foreign culture. They could coast on the grooviness of it all without the burden of racial appropriation accusations, singing about pickin' cotton and brown sugar tastin' so good with nary a care in the world. (On the other hand, they did promote and revive the careers of many an old bluesman, so that was good.)
Ironically, the British invasion rock bands is often credited with edging out the popular girl-group sound, one in which black female voices were forefront, albeit in a relatively neutered way generally controlled by male record producers. All the original members of famous girl group The Crystals, for example, were completely replaced by Phil Spector before their big hit, He's A Rebel, was recorded. Considered interchangeable, group members often had little power and rights to royalties. Similarly, in Tina Turner's biography, I , Tina (so great, BTW, though very depressing in parts), she describes how Ike constantly fired and hired an endless parade of Ikettes.
One of my fav tracks featuring black female backup singers is Betty Davis' Game is My Middle Name: The Pointer Sisters and openly gay artist Sylvester roar through their solos, doing so much more than just chiming in to back up the main attraction. Of course, there are plenty more examples- and even in some of the worst cases, the backup singers are often the best part of the song.
TINA AND IKETTES PHOTO COURTESY docleham.wordpress.com