Hello,
Black stains are usually due to iron supplements.
It could also be due to improper brushing.
Either way, if they can be cleaned with a professional cleaning at the dentist.
Signs of early caries can also be sometimes mistaken for stains.
Kindly provide images for better guidance.
Next Steps
Visit your dentist to guide you better.
Health Tips
Check any medicinal supplements containing iron. Use proper brushing techniques to clean her teeth.