The general view that Blacks/women can't be racist or sexist is not prevelant in either group. But the point of view is often used to get noticed. I too was in an AA studies course about a decade ago and this topic was discussed. But not one student or teacher truly believed it, it was a jumping off point to discuss the effects of racism and how they disproportionately affect people. If you have been marginalized in society in the past, any new racism is added to a rich historical scaffolding that our current lives are built on.
Before I was 14 I never heard a Jewish slur and when I finally heard one it was out of context, had no meaning to me and simply seemed funny in a pathetic way. Over the next decade I heard several more slurs. Each time I got madder because context was being formed. Enough times and you realize there is a real community living among us that hates Jews.
Now I can extrapolate from my personal experience to being a Black person where instead of first hearing a slur at 14, you moght hear it at 5. Instead of hearing slurs 4 times in my life, I might hear the slurs dozens of times. And you see other Blacks often portrayed stereotypically in movies, TV and media. You see jails filled with othe blacks. You get pulled over more often and treated with more suspician by police than you white counterparts. The historical scaffolding is much greater, the history of dealing with racism is much greater. So when a Black person is subjected to racism it has a far greater effect than to a Jew or especially an average white male.
The racism is real and bad from both actors, but the effects are wildly disproportional. Same with women who constantly hear quips and statements about how they can't do something or don't belong in certain roles.
So yes, a Black person can be racist, but in most cases the effects are mild comparatively. Of course there are always egregious acts, violent ones, ones that hurt deeply, and those are felt tremendously from all recipients, but most more mild acts are simply not equatable just because the actions are similar (calling someone a cracker vs N-word). In both cases the original sentiment might be equal, but the effects are not.