Hello The black line that you are seeing on your teeth might not be a cavity. It might just be a stain that you have acquired through daily dietary items. The pits and fissures on the teeth tend to attain these stains due to food items, but mostly they are self cleansable and if you maintain your oral hygiene it does not harm you. But sometimes they may progress and take the form of a cavity. So in both the cases just go and visit your nearby dentist, let him check and decide whether cavity filling is needed or not or else you could just upload the photographs for a better clarity and guidance

The black line you’re talking about could be a cavity or just a stain. It is advisable to get cavities filled as soon as you see any sign. On the surface of the teeth, the enamel is the hardest so a visible thin line or spot could easily be a deep cavity. If left untreated, you may need a root canal treatment. In case it is just a stain, you don’t need any treatment.