I recently heard from a customer who told me that his tuning forks had turned black. I thought he was being a little dramatic. Tuning forks may oxidize which causes dark splotches on a fork but completely black? Not likely, or so I thought until I he sent me a picture.
Oxidation usually occurs near the ocean and is hard to avoid. It can also occur if you leave your tuning forks in a space where there is little or no air circulation. I have lived by the ocean for years and have seen the effects. What I do is leave my tuning forks either out of the bag or open so that air circulates around them. The person I mentioned does live by the ocean and kept his tuning forks in a bag in a closet for over a year so the severe oxidation he experienced was not surprising. Allowing air to circulate around your forks is important.
If you do see dark splotches on your forks you can try using a mild abrasive scrub brush. I use one of those sponges with a soft abrasive side which you can get at any grocery store. You can put some acetone on the sponge which may remove most of the oxidation. A light scrub of the fork will not effect the tuning.