Why has my ear done this?

I have had my second ear piercings for about 10 years and this is the first time this has ever happened. My earring was completely silver, but now the area that goes through my earlobe has turned almost black. I wear two sets of the same earrings in both my first and second lobe piercings almost every day (I never mix them up, for some reason I'm weird about that). This happened in both ears, second piercings only.

What can cause this?