I had small cavities between my teeth, according to x-rays and one dentist. He said not to fill them right then, but if they got bigger, we would need to. My teeth hurt all of the time, and I was using sensodyne every time I brushed my teeth, twice a day. I switched to Weleda's calendula toothpaste, and my teeth stopped hurting. Two years later, at a new dentist, with new x-rays, my teeth showed no damage. In that time, I had also started cal/mag supplements. And went completely dairy free, but maybe that's a side issue.
Not all black spots on teeth are cavities, by the way. I've got dark stains between my molars, but they're from blueberries, not cavities. Peroxide would make the stains go away, if I swished it regularly.