I don't think so. She did the hair, the outfit and she wanted to go the 'extra mile' and try and darken her skin. Was it on the mark? God no. Does this have anything to do with blackface? I don't think so.
As far as I know about blackface, it was because they didn't want black people acting in films, but wanted to portray black people in films. They also characterized black people 'comically' with their big lips and everything else. It was offense because it was a generalization, and it was offensive because it was indicative of the position black people were in back then.
However, someone darkening their skin because they want to portray someone with dark skin for halloween doesn't seem to fall in the same category. One, because this isn't an attempt to 'take the spot' of a black person. But also because there doesn't seem to be any 'characeturization' here. She tried to be as close to the mark as possible, and it was out of respect and like of the role, rather than out of a desire to portray this character as some... uncle tom or thief or slave or whatever.