Hello dear, I have gone through your question , what I feel is that it can be Fixed drug eruption , your description matches this condition , soreness initially which turns to black spot , same site on two different occasions. it is a type of drug reaction, that occurs on the same site as previously affected , although new areas can be added to existing areas.mosyly to painkillers, tetracycline, erythromycin, penicillin. So just look back and try to figure out whether that was after taking drugs or what. Thank you