The Rosa Parks example is a poor one, as you're talking about a person whose skin colour was pivotal in who she was and what she did. That's the difference, most black people of historical significance are so because of something connected to their race. If it was an historical figure who just happened to be of a different race, in a similar style programme to this, I'd personally have no problem. I'm sure others would, just as there are some with a problem about this casting.