As a doctor I have witnessed 1000 of patients with black spots on teeth.
Prime reason is delayed and unusual schedule of teeth brushing cause sweet tooth.
Another prime reason is smoking.
Yesterday I had checked a patient with abouve said symptoms
When checked completely ,diagnosed with lungs cancer

Mouth cleaner,and brushing along with sweet reduction will improve dental and mental health.
One of the most common reasons for black spots on your teeth is tooth decay or cavities. This occurs when bacteria feed off the sugar in the mouth, producing acids. They break down enamel and cause small holes known as cavities. The cavity weakens, leading to discoloration that appears as dark spots
Thank you for reading