Petroleum jelly, such as Vaseline, glows bright blue under a black light. Anything you paint with the petroleum jelly will glow blue under ultraviolet light. Petroleum jelly contains substances called phosphors. A phosphor absorbs radiation and emits it as visible light. So the phosphors in the jelly are absorbing the invisible ultraviolet radiation from the black light and emitting visible light. If this isn't working for you, try getting a different brand of petroleum jelly, and/or using a different black light.