Special effects/post processing to make a black-colored light
I've got another idea, but it's a little kooky.  You light the WHOLE SCENE with spotlights - the fewer the better.  On the key spotlight, you add a gel that has an opaque black circle on it.  Then as you move that around, that section of darkness will pass over parts of the set/props/actors.

For a more focused spot, you could use a bright digital projector to project the image of a black circle on a white field.  Focus it based on the parts of the scene you're moving it across.