Well, Otis Blackwell's name pops up a lot when talking of singers who influenced Elvis.
Otis really should be more well known. He wrote the following hits for Elvis: All Shook Up, Paralysed, Don't Be Cruel, Return To Sender. And, he also wrote "Great Balls Of Fire" and "Breathless" for Jerry Lee Lewis, AND the wonderful "(You Give Me )Fever" for Little Willie John (and, of couse, the song was famously covered by Peggy Lee)
   Lots of Elvis's critics say that Elvis merely copied Otis's song demo - including vocal styling - when he ( Elvis) made his famous versions. But, again I would say Elvis's vocals are more interesting, more dynamic, than Otis's take on the songs.
The trouble with saying that Elvis ripped off black performers is that Elvis most definitely put his stamp on his "cover" version and certainly didn't copy the original. A good example of that is Carl Perkins "Blue Suede Shoes". Now, I know Carl isn't black, but the same thing applies. Elvis took Carl's song and made it more dynamic. Not, better, you understand, just very different.