I have an idea that I have never tried. If your going for black paint it on black, highlight with codex gray on the middle of the raised areas but always keep the edges black. Then go over with an ink but just painted on the tattoo, a black ink if you want black.
I also seem to remember a GW tutorial on putting blue tattoos on fantasy dwarfs so that could help you out. I don't have a link though.
And finally, is that a blood elf mage?