Blossom Rock was born on August 21, 1895, in Philadelphia, Pennsylvania. She was the second of three daughters born to Anna May (née Wright; July, 1871 – May 16, 1947) and Daniel MacDonald (April, 1869 – August 8, 1924). As a youth, she performed on the vaudeville stage with her younger sister, Jeanette, and later with her husband, actor Clarence Warren Rock, whom she married in September 26, 1926 Manhattan NY CN # 24447. They had no children. Rock had an elder sister, Elsie MacDonald, who had also been a vaudeville performer and then operated a dance school until 1962.

Rock adopted the name Marie Blake for her film career, starting in 1937 with an uncredited appearance in My Dear Miss Aldrich. Her first credited major part was Love Finds Andy Hardy. She became better known, however, as Sally, the hospital switchboard operator in the Dr. Kildare film series. She once had the same agent as Irene Ryan, for which she later fired him due to the similarity of their careers.

Rock returned to using her real name in the 1950s, and later gained her biggest fame by playing "Grandmama" on the ABC series The Addams Family, which ran from 1964 to 1966. In October 1964, she made a public appearance at a haunted house at The Children's Museum of Indianapolis.

Rock suffered a stroke in the 1970s that affected her speech and prevented her from reuniting with her fellow Addams Family castmates for the 1977 television film Halloween with the New Addams Family, but she watched the film from the Motion Picture & Television Country House and Hospital. She died on January 14, 1978, in Los Angeles, California, at age 82, and was interred in Forest Lawn Memorial Park in Glendale, California.