Blake Lively -- Butt Out!!! (PHOTO)

Blake Lively has pissed off the black community in a big way ... by captioning this pic that she now has an "Oakland booty."

Blake, who's pregnant with her second kid, is in Cannes and pretty clearly was just joking with the Instagram post, but it's inflamed lots of people on social media.

Blake was clearly gabbing a line from Sir Mix-a-Lot's song, "Baby Got Back (I Like Big Butts)."

Some of the people who are throwing shade think Blake is using black women as a punchline.

Notably, this doesn't seem to be simply a black/white thing. Katy Perry and Khloe Kardashian have used the quote on social media ... without getting ripped.