Black lights emit most of their light in the Ultra Violet part of the spectrum, a light that we cannot see. Some materials (like petroleum jelly, laundry detergent, and highlighters) absorb these frequencies of light and re-emit them in other frequencies which we can see, and makes them appear to glow.
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 which makes it glow.