@SarahGen-- Do you have a dark skin tone? I think those with dark skin tend to experience dark underarms, that's the case with me. My hair is also very dark and even after I've removed underarm hair, the roots can be seen through my skin which gives my underarms a dark appearance.
My underarms are doing better lately though. I use to shave and now I wax my underarms which has made a huge difference. Waxing pulls hair out from the roots.
I'm also exfoliating with body scrubs and lemons. I exfoliate my underarms three times a week with a body scrub made of coarse brown sugar. It removes all the dead skin that causes darkness. I then take half of a fresh lemon and rub it into my underarms. Lemons lighten skin because they contain vitamin C.
You might want to try these tips before buying underarm whitening creams. Those creams are made of chemicals so I don't think they're safe.