On the PhysicalSurface root you'll see a scatter group value at the bottom. Change the mouth interior material scatter group to a different number. So if the Face material has a scatter group as 1 change the mouth interior material to scatter group 2. 

If you are using the EZSkin derived Scatter/Blinn mats from La Femme, you'll see a scatter node amongst the various connected nodes of the set up. Open the scatter node in the mouth interior material and likewise change the scatter group number to a different value than that utilized by the face material. So if the face is scatter group 1, change the mouth interior to group 2.

Note also that Ben figure has a teeth material group, though the relevance is open to testing.

The blue you are seeing is actually a form of occlusion. In theory similar to ambient occlusion. A shading artifact brought about by the proximity of mesh surfaces. I've found that sometimes the scatter groups only serve to lessen the occlusion artifacts, rather that fully eliminate it. I've also noticed that varying lighting conditions lessen or increase the occlusion effect, or at least that's how it seems. In particular with HDR lighting, one image producing much pronounced artifacts as opposed to another producing much less. All else fails you can try raising the scatter scale value also.... in the Poser surface scatter node formula it appears that the artifact begins to be much pronounced at scale values 1.5 and lower.