C.V.
I have fair skin and naturally dark hair. So even after I shave it still looks dark under my armpits! Ughhh I hate it! It's not the skin though in my case it's the dark roots of the hair that show through. Gross.
But the only thing I can do is avoid reaching up to the sky LOL.
It wouldn't be a bad idea to use deodorant, I mean not just for hygiene purposes, but the solid kind might help conceal some of the darkness.
You could try an exfoliater and scrub every time you take a shower to remove the dead skin cells and hopefully the dark layer of skin.