I also had those stains on my teeth (had 9 fillings replaced) and several had the "stains". On the website of my dentist he has a slide show of many of the teeth he has replaced and even comments that the stains are "in the tooth". When I asked him about it during the work he was doing, he said unless the tooth is removed the stains remain because it cannot be helped, the metals corrode and seep into the teeth. He said the amount was not of any significant value and not to worry about it.