Copied from an older comment of mine:

"Based" (corruption of base head - from someone who smokes base - street name for crack cocaine) was popular as an insult in rap / African American circles in the early 00s

Rapper Lil B got called it and decided on a whim to pretend the meaning was changed to mean something positive, started using it in this way, it caught on - mostly through the new York scene and its attendant twitter following

As all slang does in the last ~100-150 years, passed from black people to everyone.