These lights plug into a normal 240v wall socket, and produce UV right rather than visible light.
Black lights causes an interesting effect where only white items (such as people’s clothing) glow in a fluorescent way. Because they don't produce visible light, they won't light up the rest of the room. The rest of the room needs to be dark for the effect to work.
You can also buy fluorescent paint to make items really stand out.
The key to an exciting and impressive light show at your party is to cycle through a variety of effects rather than just have the one lighting effect going all night, or worse, several lighting effects on at the same time in an uncoordinated fashion.
One set of blacklights will illuminate a medium sized bedroom. If you use them in a larger area, you might need 2 or more sets.