I can't speak on DC but in Marvel, there were basically no black people until the late 60's and then it seemed to be a totally opportunist move. Look at Spider-man, you hardly see a black person during the Ditko-years and then all of a sudden Robbie Roberson comes in. That wasn't until maybe 66 (67?). When X-men went to a beatnik bar in one of the very first episodes (64 or something like that), the free-jazz band was white (in the real world, you could count the number of white, american free-jazz musicians on one hand in the 60's.

After the death of MLK and all the riots etc., Marvel was quick to point out that there were black troublemakers too and never wanted to put the blame solely on the racists in most of the race-related episodes-I'm especially thinking about the Avengers episodes where Black Panther, the Avengers and a female soul singer (?) battled a racist group called serpents or something like that but also the "Trouble on campus" episodes in Spider-man and several other ones. Also note that Black Panther distanced himself from the Black Panther movement...