Lena Horne is probably (if not THE) one of the most prominent and popular examples of a glamorous black woman being underused in Hollywood for just that--being a glamorous black woman.
Besides casting her in the few all-black cast musicals they produced and in specialty numbers that could easily be cut out in the South, MGM had very little idea what to do with Lena Horne. Physically, she didn't fit the stereotypical (but always available) servant role; there were no black Leading Men in Hollywood at the time, and of course they couldn't cast her alongside white Leading Men.
Ms. Horne was disheartened by Hollywood and its treatment of black actors/actresses and switched her focus to singing.
Below is Lena singing "New Fangled Tango" from her live album Lena Horne at the Waldorf Astoria--recorded and released in 1957.