Why are armpits darker than the rest of your body. I’ve noticed this on a few women. Their armpits are darker and somewhat dirty looking. Whereas others have relatively clean looking armpits. Is it the deodorant affecting the skin somehow?
5-oclock shadow?
Hair stubble?
Wow–great minds think (and simulpost) alike! 
Well, both the armpits and the err… naughty area… have special sweat glands. Maybe this is the cause?