Hi You should consult with your nearest pediatric dentist for treatment of the decayed teeth and maintain good oral hygiene to prevent spread of caries

Dentist need to clean the blackish area and fill it with permanent cement.

U consult with me on vedio consultation

If their is a blackish spot on tooth which doesn't goes away with brushing then it might be starting of cavity on tooth. Consult with doctor online or offline for better guidance and treatment options. Don't brush too hard on that part as it can worsen the situation.

Dont brush too hard, it will just worsen the situation,  apart form this brushing too hard might cause sensitivity and damages the gum. Better to visit dentist only, dont try something for which you might regret later, At least have a consultation with some doctor and know why that black colour is happening in the 1st place.

Please visit a dentist to get a check up done. If it's not going by brushing it's likely you can do anything to remove the stain/decay at home. A professional can help you with managing the issue.