Hey I have a weird question, mostly out of curiosity, how would one go about creating black light.
It would work as light, but would emit the color black…
Just change the color of the light to black
The way our eyes see a real life black light it appears as a really dark purple color, so that will be where you want to start.
Setting the light color to black will prevent the light from emitting any light at all, so almost black with a hint of purple will get you closest to an actual black light.
Here’s what I did as a quick test, try out the settings shown in the details panel:
A black light in that sense doesn’t actually emit black light but rather ultra violet, the name suggests it emits black but this is not actually the case. The OP asked for a light that emits the color black, obviously in reality this falls way outside the human visibility range, therefore my answer still stands.
Agreed, wasn’t trying to say otherwise. It’s just in UE4 setting the color to black is the same as setting the intensity to 0 or disabling the light, it isn’t very useful on its own. 
Could OP mean a negative intensity light? You can do it Maya where you set the light intensity to a negative value and it acts like a light vacuum, sucking up all the light in it’s attenuation sphere. In essence creating a shadow area where there’s no geometry.
I dont mean the ultraviolett black light in reality. Closest thing would be as kirk says, a negative light, creating darkness (or… the color black).
Instead of emitting light you would emit… shadow?
Apparently it’s possible, as said by Kirk it’s called a Negative Light.
Never tried but the official answer is to do it through a simple blueprint, as negative values is not authorized by default: [Request]:Negative Lights - Rendering - Unreal Engine Forums