Large lump (infection) under bottom front teeth, took antibiotics and now it's turning black. its painful and swollen. why is it black?

Bleeding: There may be some internal bleeding in that area. The teeth need to be treated by your dentist ASAP.

See a dentist ASAP: Any other treatment like NSAIDs or Tylenol (acetaminophen) OTC, antibiotics is a temporary relief.The tooth infection you speak of can may spread to your jaw and to other areas of your head and neck. I would strongly encourage you to seek treatment at your dentist ASAP since those complications can occur at anytime. Take a good care.