If i have a very tiny black hole in a tooth that is very reachable by toothbrush is it a cavity? how could this have happened?

Maybe: It may or may not be a cavity. Sometimes a fissure line or spot in the enamel may appear black. Are you having sensitivity to hot, cold, biting or sweets? It also depends on where the spot is located. Too vague- if in doubt, go to dentist and have it checked.

Stained pit: It is not uncommon to have grooves and pits in teeth , especially the back molars. Sometimes these grooves or pits can retain stain from the foods or fluids in the mouth. Some of these spots can be cavities without any discomfort. Most are just shallow defects that will never become a dental caries. You should have it examined by your dentist to be sure. Good luck.