if you have taken any iron suplements in past few days then they may lead to such kind of black stains on teeth,
some times they may be just physiologic process due to bacterial actions on teeth.
excessive use of tea or coffee or any mouth freshners also induce such stains.
Next Steps
visit dentist for scaling appointment and get them removed in less than 30 mins.
Health Tips
also check if any cavities are there or not, sometimes there might be chances of it.