I used to be really into cosplay and the cosplay community and these kinds of questions about makeup and skin color would come up regularly. And without fail, the consensus always ends up being that it's a bad idea. The rule tends to be that you shouldn't change your skin color unless it's a fantasy color like purple, green, etc. It very likely will be offensive to someone. If you're okay with offending people, that's your choice to make. But if you do this, especially at a convention or if you post it online, there will very likely be people who are bothered by it. The fact that you felt the need to ask usually means that there will be at least a few people bothered by this.
This is a bit of an atypical situation though since you're black and you're portraying the character as black. You're not portraying a different race than your own. I think that's what bothers most people. Especially if it's white people mimicking a different race.
But although this isn't mimicking a different race, it is mimicking a real skin condition that people can be born with and it can be a struggle for them. People may see that as insensitive to their struggle.
Personally, I don't think your costume idea is a major offense, and you probably won't get too much backlash for it, especially considering that you are portraying yourself as your own race. But some people will definitely see it as an offense. If you don't post it online, and you're not going to a huge convention, it might not be an issue for you at all.
But if I was a person with vitilligo, I might be a lot more upset by it, especially considering that the character is a horror character meant to be scary to look at. Imagine having this disorder and running into someone at a convention who is wearing the thing you're insecure about to look scary. It might be a bit hurtful and make them feel worse about their condition if it's something they struggle with or have been bullied for.
Unrelated to your issue, but I saw another comment in this thread saying that portraying black characters as a cosplay is different than doing Vaudeville-style blackface, but most people in the cosplay community would disagree with that. Nobody has a problem with people playing characters who are a different race than the cosplayer, but the consensus tends to be that you shouldn't change your skin color to do it. It's heavily discouraged. "I can't take off my race at the end of the day" is something I've heard from black cosplayers a lot when this discussion comes up.
I've seen white people cosplay black characters in full makeup at conventions and it doesn't go well. A white woman cosplaying Michonne From The Walking Dead in full makeup was the talk of a con I went to once. Lots of "oh my god, did you see that? Why would she think that's a good idea?" Changing skin color makeup is definitely a big deal in the cosplay community, and a lot of people have zero tolerance for it, even for this kind of thing.
Anyway, this has gotten very long. I'm not going to tell you what I think you should do, but I wanted to share my experiences with this. I don't think your intentions are hurtful. But if you choose to do the costume, you need to be okay with the fact that it will very likely offend or be hurtful to some people who see any skin tone changing as completely unacceptable. Consider where you'll be wearing it and the kind of people who might be there and make a decision from there. I don't think it's really very offensive, but I'm a white woman and I don't have vitiligo so I can't speak for everyone. I have a few mixed feelings on the issue, because it's complicated. But personally I would rather avoid doing something potentially hurtful, even if it's unlikely to happen, than risk upsetting someone or making them feel bad about themselves because of what I chose to do.
I hope that helps you make your decision, and I wish you good luck no matter what you decide.
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