In cycles, I just learned, that you can set a lamp's value to a negative number to create a "negative lamp". But in a few tests, instead of removing light, it seemed to just cast black light? What are the lamps actually doing?
Regardless of what render engine you are using (either Cycles or Blender Internal),If you are using a point lamp. it is actually removing light, because it is impossible for a lamp to cast black light. Set a lamp's emission color to pure black. and set the strength as high as you want, if you have any other lamp in your scene, emitting light of any other color, with the strength any higher than 0.00, your scene will not show any traces of "black light" If you are using an emission shader, set to pure black, then yes it is emitting "black light" Lamps can not cast black light, but an emission shader can, and does.
You should also note that there is a difference between setting a lamp to black, and having a positive strength, and setting the lamp to white and having a negative strength.
When using an emission shader, there is also a difference between emitting black light and having a negative lamp.
In this file, the black cube is emitting black light, the white cube with negative strength is literally removing light. Both appear to be casting a shadow. the white cube with positive strength is emitting light.