Hello
Whatever your dentist has told is absolutely correct. It can definitely be a stain or it can be a pit caries. If it is a stain it will go away with your regular toothbrushing and oral hygiene maintenance, just give it 10 days if it does not go away or does not fade with time then it might be the initiation of some cavity you can go and get it filled thereafter
Health Tips
Do not brush forcefully over that Tooth leading to damage of the tooth