Vocally trained from an early age, Blyth began her career as a child performer on radio and was singing with the San Carlo Opera Company while still a youth. By age 13, she appeared on Broadway in the dramatic role of the daughter of Paul Lukas in Lillian Hellman's "Watch on the Rhine". Signed to a Hollywood contract by age 15, Blyth found herself in fluff such as "The Merry Monahans", "Chip Off the Old Block" and "Babes on Swing Street" (all 1944) before "Mildred Pierce" positioned her for a dramatic career. Unable to capitalize on the Oscar nomination because of a back fracture, she did not reappear on the screen until 1947. Now cast in dramas, she was particularly effective as a young Regina Hubbard in the film version of Hellman's "Another Part of the Forest" (1948). The character was not a poster child for sweetness, to be sure, as she joined her siblings in turning on her father. By the early 50s, Blyth was back singing for her screen time in musicals such as "The Great Caruso" (1951), the disastrous remake of "Rose Marie" and "The Student Prince" (both 1954) and "Kismet" (1955). With her sweet operatic voice, Blyth was oddly cast in the title role of "The Helen Morgan Story" (1957), based on the life of the torch singer--and, in fact, Blyth had to have her songs dubbed by pop singer Gogi Grant. (The film was eclipsed by Polly Bergen's successful TV version of Morgan's life.) Whether it was because of the waning days of musicals or her desire to raise a family or both, Blyth retired from the screen after the release of "The Helen Morgan Story" and to date has not made another film.
Blyth did not, however, retire from TV, which could offer a quick performing fix through one-week's work on an occasional guest appearance. She had made appearances on the small screen in the 40s and 50s, including 1953's "RCA Victor Show" and a version of "A Place in the Sun" (1954). Beginning in 1958, Blyth began regularly making guest appearances, six in that year alone. She would sing on variety shows, such as those hosted by Dinah Shore, Eddie Fisher, Ed Sullivan, and Perry Como, or play an occasional dramatic role, such as on several episodes of "Wagon Train". Blyth also made a memorable "Twilight Zone" in 1964, as a woman who had found the Egyptian secret of eternal youth--the bite of a scarab --and whose age is only given away by her daughter--a shriveling old sourpuss. She continued doing episodics into the 70s and 80s, appearing on such shows as "Switch", "Quincy, M.E." and "Murder, She Wrote". Together with her children, she did TV commercials for Hostess Twinkies, cupcakes and fruit pies in the 60s and 70s.