How can I cast artificially darker shadows? I want to make certain shadows more prominent (using Cycles) without further illuminating the rest of the scene.

Example: I have a simple studio with a bowl of fruit and 3-point lighting.  The light looks great so I don't want to change it but I want the bowl to cast a darker shadow on the ground (without dimming the back-light).

Alternatively, and this would be really cool, an anti-light source.  Like a spot light that throws darkness instead of light. (Am I crazy for wanting this?)

Thanks!
You have three options.


You could add a light that is only visible to the shadow path, so it won't add any light, but it will cast shadows.  If you're happy with the scene otherwise, you should duplicate one of the existing lights and move it forward or back just a touch.
Another option is to add a "negative light", where you add a light with a color of white, and a negative value for the strength.  There aren't such things in real life (except maybe black-holes) but for some lighting effects it is a neat trick to add darkness.
Lastly you could do something with render layers, and render passes to get the shadow on it's own render layer and then control how dark it is during composting.