Let's go back to the 60s Civil Rights era shall we? By the 70s, black men are viewed as a physical "threat", and coincidentally this is when black on black crime exploded. And a lot of media--- "blaxplotation" films glorifying the angry Negro who was "fighting THE MAN".
Before that, black men were viewed as "yella", even in the early 1900s, a white man would not box a black man in the ring because he was view inferior, and didn't know how to fight (or the white guy in questioned didn't want to be beat.)
Even Jack Johnson had to fight in the basement.
So think about this, before the 1960s, Black men were thought of as easily controlled, non-violent push-overs. And by the 70s, they are thought of as uber masculine, uber violent criminals.
My, how the powers that be can change societies perception through media and marketing.