This could be due to many reason like say: Shaving,dead skin,Antiperspirant and deodorant usage or some medical condition like Acanthosis nigricans or Hyper-pigmentation.please visit your doctor to rule out any cause..
For remedy you can stop shaving or waxing,and change your deodorant and if its due to medical conditions using Retin-A, 20% urea, alpha hydroxyacids, and salicylic acid might benefit for lightening your armpit.but i would suggest you to visit a dermatologist before any treatment..