Hello So there are pits and fissures present on our molars that aid in eating food and these have a very major tendency to take up the stains of the food items that we eat and drink in the whole day. But these are self cleansable. Only if we are not regular with our oral hygiene regimes these can develop cavity which can progress further. So a clinical examination by a dentist would reveal whether these are just stains present in the pits or a cavity has started. Since you have mentioned that your dentist did not tell you about it so they must be only stains but if you still have doubt you can visit your dentist again and ask him to examine again.

Hi, Happy to see that patients are so aware about their oral health.Black lines on teeth can be due to stains or initiation of decay. Decay can be determined only on physical examination or through xray.Visit and ask your doctor to examine these teeth in particular.If black lines are just stains thn  its normal and scaling,polishing will be helpful.