Hi, I have dark armpits as well, I am Indian and my Doctor said it is common due to my complexion. I've tried Sandalwood powder as well as Turmeric, I just added a little water and created a paste, I applied it twice a week for about 3 weeks until I noticed a difference. I've also noticed, the hair makes it significantly darker so I make sure to keep them hairless. There is also a Dove roll on that has been helping as well as a Garnier roll on. I thought the Garnier would be more useful as it is the more expensive one, however the Dove works fairly fast. Also, if you use powder in your underarm region, I'm unsure as to why, but this worsens the pigmentation.
By "dark" do you mean that the skin is dark, or the hair?
If it's the skin, it may be a result of the clothes you are wearing.