Born in America to Swedish-American parents, mezzo-soprano Blanche Thebom was among the select number of American singers who grew into major careers at the Metropolitan Opera under the management of Edward Johnson. The attractive young singer found opportunities at the country's leading opera house due in large measure to WWII's stopping the supply of experienced artists from making their way to America during those years. Unlike many others, Thebom was sufficiently gifted and well enough prepared to turn opportunity into an enduring career, making herself a valuable company member for more than two decades.
After her family moved to Canton, OH, when she was still a girl, Thebom began singing in a church choir and participated in a school performance of Flotow's Martha. Financial hardships within her family obliged her to seek employment as a secretary, while still performing occasionally at weddings and in various church choirs. When she traveled with her parents to Sweden in 1938, she was invited to perform at a ship's concert. Also aboard the Kungsholm was Kosti Vehanen, then accompanist to Marian Anderson; impressed, he informed the young singer that she should undertake serious study and plan for a career as a professional singer. When she returned to the United States, Thebom found that her employer was willing to provide financial assistance and she began a period of concentrated instruction. She was able to study with such prominent teachers as Lothar Wallerstein and former soprano Edyth Walker. In addition to work on voice production and repertory, she was also trained in languages and stagecraft.
An audition in 1940 led to her being given a contract by impresario Sol Hurok. A recital debut in Sheboygan, WI, was followed in a month's time by her first orchestral appearance. Performing Brahms' Alto Rhapsody with no less an orchestra and conductor than the Philadelphia Orchestra under Eugene Ormandy, she began a year devoted to touring the States in recital, interspersing occasional appearances in orchestral concerts. In January 1944, her New York Town Hall recital brought several complimentary reviews. That summer, she appeared in the motion picture, When Irish Eyes are Smiling, an engagement facilitated by her poise, fine voice and good looks. November 1944 brought her Metropolitan Opera debut during a Philadelphia visit. Her Brangäne there was followed in December by her first appearance in the Metropolitan's home in New York, this time as Fricka in Die Walküre. Praised for both her fine voice and her stage sense, Thebom began an association with the company which would be her principal home for 22 seasons.
While pressed initially into the Wagnerian wing, Thebom gradually made her mark in the Italian and French repertories as well, ranging to such roles as Marina and Orlofsky (in Fledermaus). In San Francisco she first appeared in 1947 as Amneris; she then ventured overseas, first to Sweden where she was a guest artist for the Stockholm Opera's Golden Jubilee, and, later, to England's Glyndebourne Festival where her Dorabella was heard in 1950. Thebom retuned to Stockholm in 1956 to undertake her first soprano role, Elisabeth in Wagner's Tannhäuser, an experiment of limited success. In 1957, she was the Didon in Covent Garden's production of Berlioz's Les Troyens, where critics complained that, while dramatically striking, she was not in good voice.
Following her retirement, Thebom taught and involved herself with a school in California devoted to preparing young singers for early entrance into professional careers.