With so little information, TeenHealthFX recommends that you go see your doctor about the black marks on your breast. They could be birthmarks or a sign of minor irritation from something rubbing on your skin, such as the underwire of a bra. It is still necessary to go see a doctor because they will be able to diagnose and help you treat any problem.
If you live in Northern New Jersey, please call the Adolescent/Young Adult Center for Health at (973) 971.5199. If you live outside Northern New Jersey, please call your local teen health center.