I believe it has something to do with the historical connotation. It's a relic from slavery. At least in US, and because we are close by, Canada. Literally typing in blackface on google yielded this
Blackface is a form of theatrical makeup used by performers to represent a black person. The practice gained popularity during the 19th century and contributed to the proliferation of stereotypes such as the "happy-go-lucky darky on the plantation" or the "dandified coon".
It's become a fine line of whether blackface is acceptable or not. Normally kids going out in Halloween costumes with blackface on is as offensive as someone dressing up as Hitler. There is a strong memory of what it represents. But then you have instances of Tropic Thunder, where they make fun of the whole idea. Or in Netherlands, where it is part of tradition, and less likely to be politically incorrect because over there, it doesn't have the stigma of hundreds of years of slavery.
Now going in drag also has it's societal limits. One can parody and mock, but if they go over the line, it becomes offensive. Same thing with jokes on race.
