And just another note that the fairly dark portion of my armpit closest to my back is the skin after the healing from a large boil about a month ago.
I'm not a doctor, but I get the exact same things and was told by my doctor it is a cyst and will go away on its own. It also could just be an infected ingrown hair or a blood blister from too much sweat, or like you said maybe a recurring boil. Try putting a warm towel pressed to the affected area for about 15 minutes and maybe some vaseline after if it starts to hurt or sting.
Thanks for the feedback.. greatly appreciated. So the redness in the general area (compared to the other armpit with is not nearly as red) isn't an issue to be concerned with? I was thinking maybe this was a rash but wasn't sure.
Well if you shave in the particular area, its definitely a infected ingrown hair, if not its just a cyst. Go to your nearest pharmacy and ask for an over the counter ointment, since I can assume you probably live in the U.S. and it might be money consuming to see a doctor to give an official diagnosis.