Blackish could be decay or even stains. Hard to say without a photograph.
Please Visit the dentist.
Next Steps
A decay can be restored in the early stages itself before it progresses to a root canal. And if it's stains then you can get scaling and polishing done.
Health Tips
Avoid snacking inbetween meals.
Keep brushing twice a day.
And avoid any stain causing habits like smoking and drinking tea coffee.