I’m not sure white people in general are offended by black people talking about racism.
I’ve seen comments from white people on SM that range from total support for protesters/BLM and finally tackling racism in a meaningful way, to questioning the narrative as presented (AKA “but was it racist? I get asked x, y and z all the time”), to absolute denial that there is a problem.
I have also seen some very extreme/negative comments against white people in general on SM, which, personally I find disheartening and often reduces the discussion to petty “oh, so its ok to say that “all white people are xx” but racist to say “all black people are xx” comments, rather than meaningful dialogue.
Personally, I think people just don’t like being confronted with their own prejudices and go into defensive mode. Also, it’s hard to see there’s a problem when you aren’t on the receiving end of it. As a PP has put, I have given up talking to men about sexism because they do not get it. It always descends into “but women are given equality under the law, men have to fight in wars, men are more likely to die of suicide than women”. None of which I deny but it doesn’t take away from the day to day experience of being on the receiving end of sexism.
I’m not saying it’s truly comparable, just that it’s very difficult to make anyone see outside of their own personal experience, hence the offence and defensiveness about tackling such a subject (particularly when coming from their “accusers” so to speak).