This is what i came up with from researching, A little late but i was browsing questions because i was wondering why mine was like that as well...........Stop shaving your underarms and stick to waxing. Wax very regularly. Your underarms will gradually regain their natural colour.
Home Remedy For Dark Underarms
The best and effective home remedy to treat dark underarms it to use lemon. Take a piece of lemon and rub it on the armpits.
Baking soda also acts favorably in curing the darkness of the underarms. After bath, apply some baking soda onto the armpits.
Apply Vitamin E oil on the armpits to treat dark underarms.
Rub the underarms with coconut oil to remove the darkness from the underarms.
Make a paste using 1 tsp of lime juice, 1 tsp of cucumber juice as well as a hint of turmeric powder. Apply this paste on your armpits and wash off after 20 minutes.
Take 1 tsp of gram flour, 1 tsp of curd and a pinch of turmeric to make a paste. Apply this mixture to your armpits and rinse after it has dried with cold water.
Mix a little sandalwood powder and a few drops of rose water to make a paste. Apply this paste onto your underarms.
Potato is an active bleaching agent. Apply potato juice on the underarms and keep it for sometime. It would effectively remove the darkness of the armpits.
I havee the same thing and i am embarrased about it and havent worn sleeveless shirts in 10 years.. I dont knwow hat it is but I just wanted to let you know you are not alone