Not sure how the artists of the time felt about it, surely it brought more mainstream attention to that music.

He was good, very good. The King I guess.

But, otherwise, many people may never have heard them at all?

I don't know that Little Richard, Chuck Berry, or even, MoTown would have been as popular if Elvis had not "normalized" the music. I give him a lot of credit.

that he had a very good relationship with black musicians and others. Would be an interesting thing to explore. I think they were more than happy to share "their" music and not worried about cultural appropriation.

I mean, I get that it's unfair that Elvis was able to make a fortune performing music while others toiled in obscurity, but that was a result of a whole bunch of factors (TV, the new teenager culture, and the increased prosperity of the 50s), and not really his fault. He was a cover artist (like most performers of that era) and performed music across racial and genre divides. The B-Side of "That's Alright, Mama" was Bill Monroe's "Blue Moon of Kentucky."

More importantly, the idea that there is "white music" and "black music" is itself an artifact of the Jim Crow era. Musical traditions from Europe and Africa have been combining and recombining in America for centuries, and most styles of distinctly American music are a blend of the two. When recording technology hit, the publishers and distribution networks created the "race music" category, and deliberately excluded music by black musicians from being sold in the shops that white people frequented. Performers like Elvis and Buddy Holly did a lot to bridge this artificial divide by creating a wider audience for that music.

...was brought to America from Africa, I have read.

Its ancestor is the akonting, from West Africa, but the banjo as we know it was invented in America. (The traditional playing styles, especially clawhammer are extremely similar to some styles of West African music). It was a staple of the minstrel show, and was popular among both black and white players and audiences through most of the 19th century, and was a part of the earliest Dixieland jazz ensembles. Around the turn of the 20th century, it had become passe to whites, and its association with minstrel shows and all of their ugly stereotypes of black rural life led black musicians to abandon it, and it pretty much disappeared everywhere except for Appalachia.