The textures on faces inside the model (or outside) when smooth lighting is on, act like the ones that should be at the very edge of the block, for example if the face is facing down, it acts like the one on bottom of the block, the one that would touch the block under it. That means if there is this block under, the texture on the face that doesn't touch this block turns dark like the one that does.

The problem only occurs with smooth lighting on, probably because with smooth lighting off the texture that is hidden by touching block doesn't turn black (but I can't be sure)