Fake it. Use your preferred compositing application to turn down the exposure value over a specified region. For example, if you're talking about a round spotlight affecting a flat region, create an appropriately shaped ellipse, feather it to taste, and turn down the exposure. If instead you're talking about volumetric lighting (like shining a light through fog or smoke), you'd use a cone shape, again feathered appropriately. If your light/darkness cone intersects objects at various distances, you'll need to model those shapes in a 3d app, matchmove the camera, and use the volumetric light source from the 3d app as your exposure mask in your compositing app.
If you'd rather not fake it, then you're limited to the real world behavior of light. In real life, "negative light" is shadow. So the trick is casting shadows where you want your negative light to appear. If it's important that your negative light appears as a beam, you'll need to introduce particles such as fog or smoke into the atmosphere, and as brettfromLA pointed out, use a "black circle" to mask out your beam. In lighting parlance, these optical elements are usually called gobos. I would guess that a single diffuse light source with a gobo would work better than brettfromLA's suggestion, though. Maybe a single china ball with a gobo, but this would definitely require some experimentation.
Remember, darkness is just a relative lack of brightness.