I need help with my foot!! Help please?

Since it's summer I've been wearing flip-flops everyday. But I think it's doing some damage to my feet. On the back of both of my big toes they are dry and hard. It's on my heels too. These areas alway get stained (like when you're barefoot and your feet get dark) and no matter how hard I wash it gets stained easily. I think it's because my flip-flops are black but I'm still annoyed. How could I get rid of this? Would a pumice stone work? And how can I prevent this from coming back? Thanks.