Hello,  I would like to mention here that,  Not every black spot on tooth is a decay.  To confirm,  you need to visit dentist, Too much brushing n flossing will not help it. While your routine Ortho treatment you can ask your orthodontist to check it.  If there is cavity,  you can get it restored.

Hi there, not every black spot in our teeth is a cavity, so don't stress out. Most common cause of black spot is external stain from any  food item. Diagnosing a cavity in our teeth is not a easy process. So visit your orthodontist just to check if it is actually a cavity. Now treatment of a cavity in our teeth involves removal decayed part followed by filling of that part of tooth to restore the normal function and anatomy. It is not possible to treat a cavity at home. You will have to visit a dentist for the treatment. If the cavity is very small involving only surface of enamel then there are some non- invasive methods for the treatment which involves remineralisation. Even  for non-invasive methods You will have to visit a dentist.

Cavities cannot be treated by home procedures. If it is a small black stain on your tooth, then there is nothing to worry. You should get it checked as soon as possible.