Firstly if you're dark haired then I absolutely recommend having your underarms waxed. I promise it doesn't hurt anywhere near as much as you think.
The next thing I recommend is to exfoliate under there. When you think about all of the build up of deoderant then it totally makes sense. That build is likely to be why your underarms look a lot darker.
Finally I advise you to remember to moisturise under there, afterall is skin too! Deoderant are pretty drying so you need to balance out your skin with some moisture and you'll feel such a difference.
I hope you've enjoyed this little post. If you don't do any of these steps then I'm sure you'll see a difference if you do.