Both blacklight and glow in the dark inks have been used for tattooing. Glow in the dark ink absorbs and retains light, and then glows in darkened conditions by process of phosphorescence. Blacklight ink does not glow in the dark, but reacts to non-visible UV light, producing a visible glow by fluorescence. The resulting glow of both these inks is highly variable. The safety of such inks for use on humans is widely debated in the tattoo community.
The ingredients in some "glow" inks are listed as: (PMMA) Polymethylmethacrylate 97.5% and microspheres of fluorescent dye 2.5% suspended in UV sterilized, distilled water. These inks are as safe as any on the market. However you are well advised to test a small piece of skin before committing to a large piece.
(according to wikipedia)