Hi Natalia, from your history it looks like you have developed a hematoma
or a blood clot just below your nipple region due to the injury
and that is why it is looking black and is itchy. Hematomas become darker the first two days but then later change colour from purple to blue to green to yellow and finally resolve in 7-10 days. So if it starts changing colour in the next few days and your symptoms start subsiding then you need not worry but if it persists beyond that then it would be a good idea to visit your general physician and take an openion about further action. In the mean while you could take an anti inflammatory/analgisic to alley the pain and do not rub or massage
that area. Wear a supportive bra which is not too tight and avoid any further injury. Hope it resolves fast! TAKE CARE.