I am mixed race and have not heard the term 'blackface' to mean dressing pejoratively as a black person before.
My family, in the West Indies, throw lots of parties and we all dress up. In the past, I have dressed up as a Dutch policeman in a blonde wig (my DH is from Holland and he dressed as Bob Marley.) We both wore wigs and lighter/darker foundation.
At the party, there were relatives with African blood, Irish and Scots ancestry and all the permutations in between.
I remember a ridiculous amount of laughter. If anyone had started lecturing us about white opression and how offended we should feel, they'd have been asked to leave or have a large drink and chill out.
One if the many things I love about my family is that they taught us to look forward, ignore the skin colour and never presume to tell another person how they should feel.
Hand wringing and lecturing, hectoring and determination to see racism where none was intended can, however well-intentioned, come across as patronising and condescending.
Please don't presume to tell me what's in my heart. I refuse to treat people differently because of their skin colour and that's how we are bringing up our kids with all their cousins of all colours.
Telling me that I am racist because I am not offended by fancy dress is ignorant and patronising.
You're entirely free to be furious about it. Just don't speak for me or spoil my party with your presumptions.