Huh. Just not sure I understand this correctly. A light cannot emit black light (in essence). However a light can be used to process shadows and then, shadows only. You can also look at the shadow only material or you could create/bake a shadowmap.
Apart from that, you’re gonna need a light (any light) to light your scene or your scene will just look plain black. Now, light only adds to light and given this. If the light that is lightening your scene is of an intensity of say 1 or 0.8 and you add a light which casts shadows with an intensity of -1 or -0.8, the light casting the shadows will add to the light with -1 value which when opposed to the +1 value of the scene light equals 0, meaning the highlights will not become brighter, only the shadows will (which if I understand correctly is what you mean when you say “a black light”, is it?
Here’s a PG where you can try casting shadows from a spot or a directional light with opposite intensity to the default hemi light that sets global lighting on the scene.
If this is not what you are seeking, may be you can try explain a bit more…
Else, have a great day
What I need is a way to project a texture, like a shadow, without surrounding light affecting the area.
See the white stripes on the ground on my PG?
They are lit, hence their diffuse color reacting to intensity.
I want them to be black or in shades of grey depending on the alpha (or any other trick, really).
I will update with a picture of what I am after exactly when back home, in less than an hour.
I am reluctant to use a shadow generator because I am aiming at a specific unrealistic lighting result.
Edit: I know about the impossibility of a black light
Edit2: I just re read your answer and there is something to play with. Keep you updated