Usually it involves scaling the teeth down or adjusting the morph below the lower lip. It's not black teeth, actually -- it's the backside of the polygons poking through the bottom of the lip. Most morphs don't include the teeth/inner mouth area, so those will stay the same. You can test it (to see if I'm right or blowin smoke) by making the material transparent for the gums/teeth...

thou and I, my friend, can, in the most flunkey world, make, each of us, one non-flunkey, one hero, if we like: that will be two heroes to begin with. (Carlyle)