Someone fell into a trap that you set up for them through provocation. I think the thing is that people can see that. And they can also see that the argument you want to have is a self-referential one which ends up being about the definition of a word, rather than a useful discussion involving complicated contexts. I think that it's possible for a black person to be racist towards a white person in the UK, but I also think the instances where it would be a significant problem are very limited. In fact that position is, in practical application, basically the same as the one you express in your blog, but you want to state things as an absolute so as to provoke people into indignant emotional reactions. It's one way to start a conversation of course but it's something many people will see and decide not to bother.
