2 things,
Persecution of blacks does not occur today. Sure, there are prejudices. But not one black person today has experienced slavery. And the youngest generation arguably has not experienced racism of any kind.
Also, Christians historically have faced persecution easily on the same level. Yet they are one of the few groups you can insult with no fear of repercussion. I do it... the fundies and the child-molestors make easy targets. Same goes for the Irish, who were considered less valuable than slaves. But how many times do you hear the argument of someone being "too white" used legitimately in a discussion.
What if you are implying that tall people are inferior?
This all comes down to being insulted, with the exception of religion, for things outside of one's control. And, in my opinion, it's just as bad but not any worse to call someone inferior for their race as it is to do the same for their gender, physical ability, or number of toes.