Multiracial whiteness doesn’t exist. It’s an idiotic (and extremely racist) term invented by Cristina Beltran to describe people of colour who don’t think the way she thinks they should.
My references to multiracial blackness were sarcastic. This doesn’t exist either. “Blackness” and “whiteness” in this context are shorthands for an entire set of behaviours and, attitudes and beliefs which it’s assumed people of colour and white people will hold. This assumption is both racist and stupid.
Hope that clears things up.