When I'm refinishing furniture, I kneel and lean a lot. You have to get in there deep if you're as thorough as I am since I refinish inside and outside for a completely clean slate.
I rounded the corner and saw my knees in a mirror and was like...um...that's not cute and was mad.about.it!
And yall know I have knee pads, right? Right.
So I went old school on them and started using the loofahs I use on my face on my knees and elbows whenever I shaved my legs and armpits.
I just gently rubbed them and probably within a month...I was back to normal. There is no reason for your knees and elbows to be black as this is something so easy to fix. And don't go crazy either or you're going to scratch yourself up and cause even more of a mess. Just rub gently and it will work. If you have a really bad problem, use lemons too. That's what an Elder told me and well...she's normally right and has perfect skin.
I love those face loofahs for weekly exfoliating my face too. Just put your face wash on and GENTLY rub your face with a wet one. Make sure you rinse the disc well and let it dry all the way so it doesn't start growing fungus.