After exporting that color map made by Blacksmith, I used it like this. When you have a tatoo on a white background, you don't even need a mask. Just multiply it using a Color_Math node with your original skin texture.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)