Mercer was found dead Oct. 29 at her home in Northridge, said Reginald D. Brown, a friend. He said the cause of death had not been determined, but Mercer had suffered two mini-strokes last year and had been in ill health.
The tall, North Carolina-born Mercer sang what bluesman Willie Dixon once described as "the real low-down blues."
"I would classify her as a classic blues singer," said Brown, who is making a documentary about Mercer. "She had a deep, powerful voice. Her idol was Bessie Smith, so she sang in that idiom."
Mercer was already singing in Paris in 1960 when she met Maurice Girodias, the French publisher of banned books such as "Lolita" and "The Ginger Man," who hired her to sing at -- and later to run -- the Blues Bar, one of four clubs he owned in a building next door to his editorial offices.
Mercer performed at the club for eight years, Brown said, with Memphis Slim on piano and Sonny Criss on alto saxophone. She also toured Europe during the summers with the Keith Smith Climax Jazz Band.
In the 1970s, after returning to the United States, Mercer had roles in the Don Siegel-directed Clint Eastwood movies "The Beguiled" and "Dirty Harry."
She also appeared in the movie "Pretty Baby," the TV-movie "A Woman Called Moses" and made guest appearances on TV shows such as "Kung Fu," "Mannix," "ER" and "The Shield."
She also was a producer of the 1972 documentary film "Angela Davis: Portrait of a Revolutionary."
The daughter of tobacco sharecroppers and one of nine children, she was born Mary Ruth Mercer in Battleboro, N.C., on June 12, 1932. She began singing in church as a teenager and ran away from home in 1947 to pursue a singing career in New York City.
The twice-married Mercer is survived by her children, Jessie Mae Frazier and Fernando Harper; her brothers, Leonard and Sam; her sisters, Anne M. Moore and Arlene Ellis; three grandchildren; one great-grandchild; and a niece and nephew whom she raised, Wilbur Mercer and Janice Sheehan.
McLellan is a Times staff writer.