Wondering what is responsible for dark underarms? Here are 4 common causes: (1) Shaving. If you shave improperly or too frequently, it can darken the skin of your armpits. This discoloration may be the result of low-grade inflammation caused by shaving. (2) Dead skin buildup. (3) Rubbing or friction. Because your armpits are extremely sensitive, clothing that feels rough or uncomfortable can cause your skin to become discolored over time. (4) Certain ingredients in antiperspirants and deodorants can be irritating and cause discoloration.