For a singer of her stature and popularity, however, Ms. Norman attracted more than her share of criticism, especially during the 1990s, when her technique grew unreliable and that earlier grandeur became a kind of grandiosity. The critic Peter G. Davis, in his 1997 book “The American Opera Singer,” was quite blunt. Her 1991 Kundry at the Metropolitan Opera, he wrote, was marred by “overbearing attitudinizing” and “exaggerated vocal mannerisms.”

Though harsh, he was not alone in this perception . As someone who had thrilled to her earlier work, I was distressed to hear her in a 1998 recital at Carnegie Hall, a program of songs by Poulenc, Chausson and Ellington with a string quartet, pianist and some Alvin Ailey dancers. The occasion was festive, and the reception enthusiastic, but Ms. Norman’s singing was spotty. What happened in those later years?

How any “attitudinizing” crept into her performances was hard to fathom, given the authenticity she brought to her artistry at her best. She writes beautifully in her memoir “Stand Up Straight and Sing!” about hearing her grandmother singing songs and spirituals, whether happy or melancholy, always “beautiful, deeply soulful and right”; Ms. Norman carried on this tradition with unforgettable spirituals performances.

Every television image of “African-Americans being run down with water hoses and chased by dogs,” she recalled, brought long lectures from her parents that “we were just as good as anyone who breathes on this planet.” Perhaps that beautiful pride eventually became a kind of hauteur, but at first it instilled determination in the young Ms. Norman, the daring to take on weighty Wagner and Strauss roles so early, as well as intense curiosity for the world beyond her homeland. So she embraced the opportunity to sing with the Deutsche Oper Berlin, then, in 1975, moved to London and stopped singing staged opera for five years to develop her voice and work on other repertory.

Maybe it was inevitable, but during the London years, Ms. Norman started speaking with a trace of a British accent that sometimes slipped into her singing. She insisted in interviews that this wasn’t an affectation, but the result of her penchant for mimicry. If it helped make her sometimes come across like a stereotypical diva, so be it, Ms. Norman felt. The connotation of that oft-denigrated term — that is, an artist willing to be demanding for a higher purpose — actually appealed to her.