It wasn't a matter of being "a love interest of white men on stage" or not--it would have been a matter of getting onto an operatic stage at all, the color-bar being absolute in opera at the time she was training as a vocalist. Oratorio, symphonic concert, or recital work was more feasible; the Black tenor Roland Hayes was a noted recitalist and orchestral soloist from 1916 into the 1970's, making recordings in the acoustic era (both at his own instigation, to sell after his recitals, and as a regular artist on the Vocalion label); in the LP era he had several LP issues. --It might interest you to know that in 1944 he was one of the soloists in the world premiere of the Missa Oecumenica by the Russian emigré Grechaninov, with the Boston Symphony under Serge Koussevitsky (a recording exists):
As a true contralto Anderson had a leg-up from the rarity of this voice type, and from the popularity of such [white] contraltos before her as Ernestine Schumann-Heink and Clara Butt. In fact it shows the entrenchment of the color-bar at the Met that neither Gatti-Casazza nor Edward Johnson after him came crawling on their knees begging her to take on Erda in the
Ring after Schumann-Heink's retirement in 1932 and Maria Olszewska's departure in 1935. Erda would have been a perfect part for her, and the closest thing they had to a true contralto in the Wagnerian wing was the mezzo Karin Branzell.
In the event I'm not sure how much an operatic career would have appealed to her--I have a hard time imagining what she would have done for instance with the hairpin-turn mood swings of Norma, about which you ask.
BTW, did you know that Stanislavski, impressed with her when she did a concert tour that took her to Russia, tried to get her to study up a Carmen with him at the Opera-Dramatic Studio? The lady turned him down, with regret.
As to Black professional sopranos in the first half of the 20th century--there had been some before Roland Hayes, but I don't believe there were any of his prominence. Aside from all-Black productions like
Porgy and Bess and
Four Saints in Three Acts the earliest prominent Black soprano I can think of was Dorothy Maynor, who again was a recitalist, concert soloist and recording artist--I hope that in the upcoming Rusalka playoffs we get to hear her lovely English-language 78 of the "Song to the Moon" from Rusalka (again with the Boston Symphony under Koussevitsky, ca 1940) which was a best-seller and for the US audience of the time their first introduction to what was then a previously-unknown aria.
Like Anderson and Maynor, the mezzo Carol Brice and the soprano Camilla Williams were frequent singers on the radio in the 1940's, and recording artists. Williams and the bass-baritone Todd Duncan were resident artists with the New York City Opera from the mid-1940's on, followed by many other Black artists.