I think the problem is that 'blackface' has a history that si closely linked to racism, or at least massively negative characterisation fo black people. As a result, though in this case the intention was far from racist, it links to a racist history that can make people feel genuinely uncomfortable. The problem is that though you might get away with this is your private life, if you are in you are in the public eye (as this lass is) you contribute to the national approach to race. I am not saying she was racist, I am saying it was a bad idea as it helps tie football to some negative images.
It is a shame, because I am pretty sure she would hate the idea that she was a racist or made people uncomfortable (especially in football, where we have had MAAANY incredible black players representing our country proudly). However i still class it as A Bad Idea because of the tension around racism and football. It is like a young lad i chatted to that was chanting about Park Ji-Sung at Man U - he defended it because he said it was OK, they were on his team, but he could not see how 50k people chanting about 'chinkies' might not be a good thing as it made racism that little bit more acceptable.
Yeah political correctness can go fuckign crazy, but moaning about PC-ness can also cover racism, or more often just not thinking through how your actions can affect others. Sister act is a fun film, and Whoopi is a good actress, I wish the lass could have worn the costume and all been well, but I think in the end racism is still enough of a problem that it is a bad idea to do this if you are representing you country somehow.
Last thing, I had a friend at uni that it as British as me, but his grandparents are from Bangledesh. Sound guy, proud to be from the UK (brother in the forces I think) as well as proud of some of his asian heritage, and we were discussing the world cup and all the England flags. I asked how it affected him, and he thought for a minute, then he said 'the problem is that i am not included in that flag. Whenever people talk about me, I am British Asian, never ENglish I was born in England, so were my parents. I've never been to Scotland or Wales, why am I British, but you are English? Whenever people talk about me it is that I am British, so when I see all the ENgland flags, it makes me feel odd because I support the England team, but i kind of feel in some way, I don't get to be part of the group." I know legally he was wrong, he is English, but I do understand that in some ways he was made to feel outside of the group I get to be in. Also I know if he was italian-Scottish he'd get to be Scottish as well as British.