Whether it be black face or Asian or native American, it is not the role of white America, which is in the majority, to dress like a member of a minority group. It is showing contempt and is demeaning. So it is reasonable that there be outrage expressed over this type of Halloween costume.
Although it has been many years since slavery and quite a few since subjugation via Jim Crow laws, it is still in terrible taste to dress up in black face. That was part of a culture that we should not be proud of and elicits bad memories of a dark time for our country.