If you're not interested in how this works, feel free to skip to the next step. Stick around if you're awesome and want to learn about ultraviolet radiation, the visible electromagnetic spectrum, and fluorescence.
Human beings can see in a very narrow band of the electromagnetic spectrum, about 750nm (red) to 390nm (violet)--do you remember ROY G. BIV from gradeschool science class? This is just a tiny chunk of the spectrum, which ranges from radio waves (longest wavelength) at the very bottom all the way up to gamma rays (shortest wavelength) at the very top. Just below the human visible sprectrum is infrared light, far infrared being heat and near infrared is what people use for active nightvision and remote controls. Here we're more concerned with the other end of the spectrum, ultraviolet light. This is the stuff that gives you a tan (or skin cancer) if you get too much of it, though we're working with very safe levels so don't panic.
An ultraviolet LED will put out most of it's light in the invisible near UV range, with it's major output around 375 nm, just outside of our range, but with a smaller output around 400 nm, or just within range. For this reason, to the naked eye an UV LED or a blacklight looks pretty dim, but really they're pumping out quite a bit of light that you can't see. Here's a chart from wikipedia
So what good is a bright light that looks dim? Well, UV light has an interesting property, in that it causes a number of chemicals to fluoresce
. Fluorescence is when a chemical absorbs light of one spectrum and emits light of a lower spectrum. In this specific case, the high energy UV photon strikes some glow in the dark paint, causing it to emit a spooky green glow (around 540nm, at a guess). Thus, spooky sun jar!