Originally Posted by monstro
I don't know, but it seems to me that it became a "thing" in the early 90s, when Afrocentricism was in and folks were walking around with Africa medallions around their necks.
A quick Google Books search turns up a number of reference to "It's a black thing, you wouldn't understand" as a t-shirt slogan in the late '80s/early '90s. The phrase was presumably around before anyone thought to put it on a t-shirt and I wouldn't be surprised if it did originate with Pryor, but the t-shirts seem to have done a lot to popularize it.
The earliest usage I could find on Google Books was from 1981, in a book titled The Victims of Democracy: Malcolm X and the Black Revolution
, but it's just mentioned as something that black people sometimes say to white people.