The New Jersey-born actress, who later married film director Karel Reisz, suffered from cancer and died on March 13.
Mark Searle, at Elliot & Thompson, the British publishers of Blair's 2003 autobiography, confirmed her death.
Blair swapped suburban high school for life as a nightclub dancer in New York, where she met Kelly, then a choreographer on the brink of success.
Blair and Kelly married in 1941 and moved to Hollywood, where he became a major star. She was 17 and he was 29. The couple divorced in 1957.
Beginning in the late 1940s, Blair took parts in "The Guilt of Janet Ames," and "A Double Life." But her movie career stalled after her enthusiasm for leftist causes landed her on Hollywood's blacklist.
"To be very left-wing in Hollywood was to work for the unions, to work for the blacks, the ordinary things that are social democratic principles," Blair told Britain's The Guardian newspaper in an interview in 2001.
Following a part in "Kind Lady" in 1951, Blair struggled to win new movie roles for several years, focusing instead on caring for the couple's daughter, Kerry.
In 1955, Blair took her most famous role, in "Marty," playing a dowdy school teacher who captures the heart of a lonely Italian-American butcher. The movie brought Academy Award nominations for both leading actors_ but Blair lost out on the best supporting actress award, though her co-star, Ernest Borgnine, won for best actor.
Two years later, Blair and Kelly separated. She rarely discussed their split in public, and refused to criticize Kelly, who died in 1996. "I have nothing bad to say about Gene in any way ... We were married 16 years and it just came to an end," she told The Guardian in 2001.
Finding herself more popular in Europe than in the U.S., Blair moved to Paris and took roles in movies in France, Spain and Italy.
Blair later moved to London and in 1963 she married respected Czech filmmaker Reisz, director of the 1960 movie "Saturday Night and Sunday Morning."
For several years, she worked mainly in theater and television and briefly halted her acting career to train as a speech therapist.
However, in 1988 - three decades after her last Hollywood film, Blair returned to the United States to star in "Betrayed" alongside Tom Berenger. A year later, she took a part in the television series "Thirtysomething."
British comedian Arabella Weir, a friend of Reisz's children, said she developed a close bond with Blair.
"She was a tremendously loving, loyal and ceaselessly supportive friend - and really good, often wicked, fun. You could talk to her about absolutely anything - nothing shocked her," Weir told The Guardian newspaper.
Blair was offered a role in 2002 in "The Hours" alongside Nicole Kidman and Julianne Moore, but turned down the part to care for Reisz, who died in the same year.
She is survived by her daughter, Kerry, from her marriage to Kelly.
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