">Put yourself in the shoes of someone black and then consider if you might be offended by the "blacking up".
"How wonderfully patronizing."
Perhaps, but not intended to be. Maybe you could draw an analogy with those "blacking up" and not intending to be offensive?
"How can anybody not black imagine what growing up in a mainly white country is like and assume what they would feel?"
I think it is usually called empathy. You're quite right though, I can't know exactly, but I can try to imagine. I think you probably could as well if you wanted to.