Ambient occlusion doesn't use lights, and instead is a system to create shadowing in areas where light wouldn't directly strike as much. So all it does for a given point on your surface is to "look around" for nearby surfaces which would darken that point. In your ambient occlusion attributes, you can reduce the Bright attribute (whitest possible point) to a light gray, or lighten the Dark attribute (blackest possible point) to a dark grey.

But better would be perhaps to put a big poly plane over your characters (out of view) to create a darkening influence, and add a ground plane too. You can also play with the spread and max distance (0 is infinite) to adjust the AO look.

And best would be to actually combine your ambient occlusion render with a diffuse render with some actual lights, maybe even an HDR image as an IBL sphere.