Black lights work by tapping into UV spectrum of light which is why they appear purple. Since the human eye can only discern colors from red to violet (anything above violet is ultra-violet), we cannot see anything outside this spectrum without aid. Black lights regularly contain UVA light (450-100nm) and do omit UVA waves, but have a glass filter which stops the majority of short-wave UVs, which are harmful to people. When these UVA rays are present via black light, we are able to see into that ultra-violet spectrum of light. This is why you can see things with a black light that you normally wouldn't see with your naked eye.
The best colors and accessories to wear to a blacklight party are, anything that's fluorescently colored. This includes green, yellow orange and pink. The main reason why some colors glow in the dark better than others is due to phosphorescence. Green phosphor is one of the best colors to absorb light (natural or strong indoor lighting), so this is why it usually stands out the best. So overall, your best bet is to wear something that's green, if you really want to stand out!
A black light is a kind of UV light, where black lights (commercial ones) emit ultra violet radiation. Commercial black lights usually come with lens and filters to help limit the amount of exposure to UVA light, whereas pure UV lights (built for sanitization of germs and disinfection) are unfiltered and can cause serious health issues with prolonged exposure.