Dark armpits are a problem among all age groups. It is not bounded by gender. Both males and females are suffering from this problem. The main reason behind the darker tone of your arm pits is due to use of deodorants and perfumes.
Some people even use razors and hair removing creams in this area. Doctors do not prescribe use of hair removing creams because they contain various harmful chemicals which can even cause skin cancer. Darkening of skin is also caused by excess and frequent use of deodorants. Perfumes are meant to spray over clothes.
Many people spray them over the skin which causes pigmentation in that area. The skin losses its original tone and becomes dark. To decrease the darkness try using a paste of Chickpea powder mixed with rose water, few drops of lemon and pinch of turmeric powder. Apply the mixture and rub it like a scrub for 3-4 minutes. Let it dry for another 7-10 minutes and rinse off.
Apply rose water or a light scented cream after it. Avoid direct use of deodorants and perfumes. Waxing is a better option to hair removing creams. Try to get your arm pits waxed. It will help you stay hair free for a longer time.