Hello There are many things which can lead to underarm darkening. One of it is contact dermatitis either due to deodrant or wax. Also donot use turmeric extract and lime. Obesity is also cause of darkening. Also it may be due to infection like fungal. Please share the pic for better diagnosis. You can use depiwhite cream once a day for lightening. Also you should go for laser hair removal this will help in both hair and darkening.