I am not offended by blackface, but I'm white, so my opinion may not matter. I think if the intent of your costume is to idolize a character it's not racist, but some will still be offended by it. I'm also not convinced that making your skin colour look like a black person's skin colour is blackface. I think that is reserved for show polish or paint, that is an entirely different colour.
Tyler Bozak did blackface last year when he went as Michael Jackson. He took a lot of flack from that.
http://www.torontosun.com/2012/10/31...ks-online-rage
Raffi Torres and his wife went as Jay Z and Beyonce. Patrick Kane and Adam Burish went as Dennis Rodman and Scottie Pippen, all as blackface, and all received some backlash. On the reverse note Dwight Howard used whiteface to go as Justin Timberlake.
I don't think there is anything wrong with any of these costumes, as the intent was not racist, but I haven't lived through any of the lives of the people that are offended.