First of all put new shaders on the models. If you are bringing them straight from Zbrush the shaders are defaulted phongs (if I remember rightly) Use blinns for now. Chances are this is the problem.
Secondly it sounds like you have final gather turned on in Mental ray. this is what it does. its basically like rendering GI, infact its another form of GI whateverthe value of the background will essentially affect the scene. So you can turn it off and use mental rays standard renderer which is like mayas own. Just use lights instead
or You can turn the color in the environment tab in the Attribute editor for the scene to a more grey color then render it out. If its only a still then make sure you have an alpha channel then you can simply recomp it in Photoshop on a white back drop.
hope some of these help you.