The reason why your teeth glow in the dark is a result of the presence of phosphors, which are substance that fluoresce when they are exposed to UV light.
Natural teeth contain these phosphors, which causes the teeth to emit a blue-ish glow under UV light.
And some types of dental restorations such as dentures or even certain types of crowns might not contain the phosphorous and won't glow like natural teeth will.
Even teeth whitening products can make your teeth glow very conspicuously.
Phosphors are materials which absorb light energy and then re-emit the light energy at different wavelengths, which makes your teeth appear to glow in the dark.
And black lights also emit UV light, and when the black light hits the phosphors in the teeth, it then excites the phosphors and causes the teeth to fluoresce and glow.
The natural fluorescence of your teeth is a blueish color and is also a characteristic of healthy tooth enamel.