Backup singers live in a world that lies just beyond the spotlight. Their voices bring harmony to the biggest bands in popular music, but we've had no idea who these singers are or what lives they lead – until now.

Unsung talent finally get the spotlight.

The history of showbiz is littered with stories of talented people left out of the limelight – valiant professionals whose unsung efforts enabled brighter stars to shine. In this excellent documentary, directed by Morgan Neville and produced by the late AM Records executive Gil Friesen, the spotlight is turned onto background vocalists, or 'backup singers’ – the artists whose voices we’ve all heard, bringing life to the hits, but whose faces we’ve never seen.

Listening to the now familiar music with an ear for the background vocals is a revelation

Those faces, it turns out, are mostly female and mostly black. The daughters of preachers and churchgoers, these women learnt their trade harmonising in choirs and singing Baptist hymns, where the focus was on 'the blend’ – the joining of many individual voices into a sweet-sounding whole. This mysterious merging is effectively illustrated in the film by a scene featuring the synchronised patterns created by a flock of black birds swooping in unison through a blue sky.

Which is not to say the women aren’t divas in their own right. Spirited and outspoken, the artists include Darlene Love (a member of '60s girl group The Blossoms and a backup singer for Dionne Warwick and Frank Sinatra among many others); Merry Clayton (who sang backup on Carole King’s chart-topping Tapestry album as well as on the Rolling Stones’ single 'Gimme Shelter’); Claudia Lennear (described by Mick Jagger with a lecherous twinkle as 'the really hot one of the Ikettes’); and Lisa Fischer, a Grammy-winning soloist who currently tours with the Stones, Sting and Chris Botti. The younger generation of performers is represented by Australian Jo Lawry (Sting’s featured backup singer for the last three years) and Judith Hill, who was booked to sing with Michael Jackson on his last tour, but ended up performing as lead vocalist at his memorial service.

Many of these artists have voices so powerful and distinctive that they’ve been on the brink – or briefly tasted – stardom as soloists. The fact that this crossover rarely turns into a successful commercial career is part of the tragedy the film explores at length. It’s disturbing and moving to see these hopes dashed, especially in the case of Darlene Love, whose career was stalled by the dastardly doings of Phil Spector. As Bruce Springsteen says in one of the film’s key interviews, those twenty feet from the back of the stage to the front can be a very long way to travel.

Listening to the now familiar music with an ear for the background vocals is a revelation. But one of the other chief pleasures of this expertly edited documentary is its assemblage of brilliant archival footage: Talking Heads’ lead singer David Byrne hamming it up with his backup singers during a rendition of 'Slippery People’; or Tina Turner and the Ikettes grinding alarmingly on stage in bum-skimming skirts while Ike reclines like a pimp; or Ray Charles pumping away at the piano beside the radiant Raelettes.

20 Feet from Stardom feels a trifle drawn out and is occasionally repetitive, but for music lovers, history buffs and students of the fame phenomenon, it’s invaluable. And to the art of backup singing, the film is a long overdue tribute.