Next one reason for dark armpit could be a buildup of dead skin. Every time you wash your hands, face or your body, you lose millions of dead cells which are replaced with new ones. If dead cells starts building up, that will sure to be looking dark. You can make use of some exfoliating products and get off the dead skins.
Chemicals could really affect the color of your skin, that’s a known fact. A few soaps, deos, perfumes or antiperspirants are potentially capable to bring in dark skin. One way is switching the brands that you are currently with, or consult a cosmetologist and get the best one for your skin.
If you got a medical condition, namely acanthosis nigricans. Which is a condition liable to bring in light brown to black spots in areas like neck, under arms, or by the groin areas. This couldbe a pop up due to some issues related to insulin production, glandular issues and are seen common in people who are obese. Few few things you can do is to watch your diet to control insulin synthesis,make use of Retin-A, 20% urea, alpha hydroxyacids, and salicylic acid prescriptions to lighten your armpits.
Hyper-pigmentation is yet another condition that could be a reason for dark armpits. Your melanocytes or the melanin producing factories might be producing much of melanin pigment, that is responsible for a dark skin color. But this one less occurs with armpits and more not likelty to be cause,i must say. You can try bleaching you skin, but it good that you consult with your dermatologist and get things fixed.