Any other possible causes you can think of?

Looks like you have a glossy node for her face.  Glossy reflects light.  My guess is that there is something dark in the back of the scene that is reflecting the darkness onto her skin.  In theory, skin "glossyness" only reflects light (not dark), but I haven't had time to play with that.  Your skin glossy is highly reflective.  So to get rid of the reflected dark color, simply reduce "specular" in the glossy node.  Default is something like 0.2, but for skin I think something like 0.001 might be nearer the target.  There is also a relationship between "specular", "roughness" and "bump" that will keep you entertained for hours.  In summary, I think that "roughness" is a bit like simulated "bump", so either one being high will kill off reflections.   If you have a good bump map (do they exist?), then roughness can be 0 and bump high.

Hope that helps.

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Creator of OctaneRender for Poser
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