Dark underarms? Let's face it, we've complained about it many times before and it really is a turn off. I have had this problem growing up and it became a hindrance, especially at times when I had to wear a dress or a sleeveless top. Being the persistent person that I am, I searched for ways to lighten my underarm and youtube helped me a lot. A tutorial from Ms. Andrea of AndreasChoice had a tutorial for it and it worked miracles for me. I wasn't really able to replicate the specific tricks but using salt really did wonders.
What I do is mix 3 tablespoon of salt to equal parts of a liquid bath soap. I scrub my underarm for 30 seconds and let it sit for 1 minute and wash it thoroughly. I love the result so I do it for my knees also.