Is wearing blackface face paint always a racist act?

I was reading this weekend about an actress, Julianne Hough (who in all honesty, I have never heard of). Her Halloween costume, warn to a party, was topped off with her face painted in blackface.

She was roundly criticized in the media for doing so, to the point that she felt that she needed to send out a deep apology to anyone she had offended.

Yes, I am aware of blackface and its historical racial connotations. Does this mean that anyone who wears blackface for any reason is, prima facie, a racist? Is the pressure to apologize for wearing blackface really merited?

Like it or not, offensive or not, blackface was part of our cultural heritage in the US for around 100 years. To proscribe automatically the use of blackface seems, to me, as cultural censorship or historical hijacking. I’m not advocating that everyone goes out in blackface for Halloween, but to my mind, it isn’t automatically offensive.

Who decides what’s offensive to whom, anyway?