Blair was born in Cliffside Park, New Jersey. Her father, William Kidd Boger, was a partner in a small insurance brokerage firm; her mother, Frederica Ammon, was a schoolteacher. Both were Episcopalians. At the age of eight, she was enrolled in the Swift Sisters School of Dance, and recalled performing before Eleanor Roosevelt in 1933, winning an amateur contest shortly thereafter, joining a touring amateur show and performing on local radio, as motivating influences in her desire to pursue a dance career. She joined the John Robert Powers modeling agency and by the age of twelve was in regular demand. She enrolled in the Professional Children’s School but, as it was not accredited, her mother returned her to her local school so that she might eventually attend college. She graduated at fifteen, securing a scholarship to Sarah Lawrence College. However, the Board of Admission considered her too immature for entry and requested she wait one year.

In the interim, Blair joined the chorus at the International Casino in Times Square, and when it closed down, worked in the chorus of Billy Rose’s Diamond Horseshoe from January 1940 where Gene Kelly was working as choreographer. “Gene fought for me. He said I could dance, and he needed some good dancers,” she wrote in her autobiography. Blair and Kelly’s relationship blossomed, culminating in their marriage in October 1941. She and Kelly remained married for sixteen years and had one daughter, Kerry Kelly Novick (b. 1942), together before divorcing in 1957.

In early 1941, Blair secured her first role in a stage play when Kelly’s friend William Saroyan chose her to play the female lead role of St. Agnes of the Mice in his play The Beautiful People at the Lyceum Theatre, playing opposite Eugene Loring, and securing excellent reviews from leading critics George Jean Nathan and Richard Watts, Jr.. Watts commented, “The gently sweetly sincere and completely moving gravity and innocence of Miss Blair’s utterly right performance is so infinitely touching and beautiful than any studied portrayal could be, that her contribution to the work is gracefully enchanting.”

Blair married Czech-born director/ producer Karel Reisz in 1963, and would perform sporadically in later years working with Costa-Gavras’ (Betrayed, 1988) and on the mini-series Scarlett in 1994. Reisz died in 2002.

In 1980, she was a member of the jury at the 30th Berlin International Film Festival.

Blair filmed scenes for Stephen Daldry’s The Hours (2002), initially playing the older version of Julianne Moore’s character. It is unclear if she was dropped or if she withdrew from the project. In 2003 she published her autobiography, The Memory of All That.

Blair died of cancer in London on March 13, 2009. She is survived by a daughter, three stepchildren and several grandchildren.