I'm no racist - far from it, but if we carry this to its logical conclusion, then what about the Voodoo tradition where you'll see people 'whitened' up. Am I not able to complain that this may well offend me, and many others?
Banjiman has made the good point. I'd add that Voodoo witches whited-up for disguise long before they had ever seen a white-skinned person. They weren't doing it to deride and take the p**s out of white folk.
As I said above this, we are learning that blackface morris has a heritage that is more dodgy than I had previously thought.