I am black too. 25% is way too high, I know this because I used it on another area which frosted almost immediately, I left it on for a bit and when I washed it off it went really dark. It turned into thick scabs and my skin was really dark afterwards, damaged. I then used lower concentrations of TCA to reverse it, which worked.
I use 25% on my face which I dilute heavily probably to 10%, I layer this. As soon as I see frosting I wash it off. After about 5 days of peeling the skin looks amazing.
I know you have probably sorted the problem by now, but for anyone else looking into doing it, start with a low percentage first and wash off as soon as you see frosting. The areas that frost usually will peel which is brilliant, but over frosting may go to deep so wash off as soon as you see it.
I am really impressed with the results
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Posted to 25% TCA Peel - Age 27, Black Female - London, UK on 19 Nov 2011