I understand that Lieber and Stoller more or less grew up with American Blacks, and had a genuine affinity for and familiarity with the Black community's music. In other words, they were writing music they really liked, and not simply trying to exploit the genre for a buck.
Elvis' "Hound Dog" is noticeably different from Big Mama Thornton's, though similar enough to have had to have been derived from it. If I recall correctly, their "Kansas City" was likewise based on a song Big Joe Turner had been doing. It appears they modified and arranged the songs, and copyrighted their version. Probably legal and basically ethical, but certainly not as clean a process as writing a song from scratch. Still, I believe that the great majority of their songs (and there were probably hundreds) were essentially original.