Hi,
Welcome to the ehealthforum and I am glad to help you out.
Dark armpits can be due to continuous sweating of the area or hyperpigmentation skin. This darkening occurs when an excess of melanin, the brown pigment that produces normal skin color, forms deposits in the skin.
Treatment involves , azelaic acid, corticosteroids, tretinoin cream, microdermabrasion and laser treatment. Please discuss these options with your dermatologist before using them.
I sincerely hope that helps. Take care.