Why don't you ask a particular Black person??? You might get a more precise answer. I don't understand that phrase too.
edit: Here, I google it for you.
It has two meanings:
Its a stereotypical statement regarding the sexual prowess of black men. It's taken to mean that once you've been satisfied by a black man you'll never want any other men.
It also means that once you go black, no white will want you back.
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