Right. People fail to recognize that blackface is a practice created by white people to entertain other white people by making fun of black people, portraying them as stupid and uncultured. While I think asking questions about what is and isn’t okay is good practice, there’s no cultural history connected to what OP is asking if he should do. That said, I am not someone with the skin conditions in question, so I’m not the one to decide whether it is “fine”.

I do want to offer the argument that you should do your best not to give people opportunities to miscontrue your intent. You are correct that, in some cases, black burn victims can have lighter patches of skin where they were burned, but this is both not universal and not an experience everyone will have had. If you’re making a cosplay that requires a bit of mental work on the viewers behalf, you probably don’t also want it to be a cosplay which could be perceived as insensitive if people fail to make those connections or put in that work.