Black anodizing turn brown under heat/light exposure. I know this from my years in paintball. The only way to correct it is to re-anodize it. I personally don't know of any anodizers who have FFL's that can do this but there are many good ones out there for paintball. It may be worth looking into. Just be prepared because many have a 6-9 month backlog. I once sent a paintball gun to a company and forgot about it. It showed up 13 months later. It was almost like Christmas.