Blackface (or any costume in which you dress like someone from another race through appropriated cultural signifiers or the use of cosmetics to alter your skin tone) is never an appropriate Halloween costume.
It is also not appropriate for frat parties, birthday parties, general costume / fancy dress parties, public events, theme nights at bars and clubs, workplace trainings, or anytime ever for any reason.
Some exceptions to this rule are as follows:
[This space left intentionally blank.]
You're welcome. Do better.