I have a model of a head. I didn't model the inside of a mouth or anything so it's just a hole in the mesh. When I render it, I can see the texture that I applied to the outside of the mesh on the inside. All I want is for the inside of the mesh to be solid, non-reflective black. How can I do this without screwing up the outside texture?
Use the "backfacing" output from the geometry node (input section).
The output will make the side of the face in which the normal is pointing equal to 0, and the other side (back) equal to 1. This can be used in a mix node.
Note: make sure that all your normals are facing the right direction (crtl+n)
Try this nodes setup.
Slight work around, but creating a face on the mouth, by selecting the edges of the mouth and press F to create faces then creating a new material of the type holdout should make it a complete black. EDIT: Some more info:
There, we have a mouth, unlike setting this to black, this not reflect any light and will stay complete black no matter what.