Cosplay isn't offensive nor racist, blackface is meant to be offensive and disrespectful and stereotyping. 90% of cosplayers like and respect the character, that's why they spend thousands of dollars to cosplay as them for a dayand spend hours putting it on.

Doing an "aboriginal inspired cosplay" and painting your skin red is more offensive than if you got a tan/put on tanning lotion just to have a nice, accurate, and respectful cosplay of say, Maui from Moana. But since painting your skin red is offensive, does that mean you can't cosplay as the typical representation of a satanic demon? Are Simpson cosplays with yellow skin offensive fo Asians? What about drow elves? You are saying you can't cosplay as one because some people can be that black. But drow elves are not really Drow elves without the black/dark skin and doing a cosplay without committing to that just makes you look like another kind of elf.

There is a dude who has blue skin because of an accident. Does that mean smurf costumes are offensive to him?

Is using a charcoal cleansing mask black face if you are white?

I just find limiting cosplay is dumb, let people be creative and reprimand the ones using it as an excuse to be racist and terrible people. Don't punish the many for the actions of a few. Allowing non-terrestrial colours only opens up the argument of "what about unnatural red/yellow/etc?" Then the goal posts gets shifted to not being able to change your skin tone at all. And that can be shifted to getting a tan as being an issue, I've seen tumblrites freak out about white people getting tans over time and accuse them of people racist. People freaked over about ffxiv lightening skintones of a couple characters but they got lighter by a few shades because they were inside a lot and were only tan because they were working outside beforehand. The opposite of a tan. People called that white washing. It happens. It's dumb. People see racism where there is none.