"Elvis got rock music from black people" is among the oldest complaints of the dreaded [Critical Drinker voice] CULTURAL APPROPRIATION! That goes back decades.
As for "every form of music in America" being black, that's brainless. I'm curious to know if he or she has some background in music, because they don't seem to know much about different kinds of music. Like their knowledge of music goes back as far as maybe the Backstreet Boys.
Plenty of standard practice music has come out of America, and doesn't jazz owe quite a bit to that very European form of music? (Honest question, because I really wouldn't know. Just guessing)