I stepped on a nail and it started to turn black around it. what's happening?

Infection: Your wound is possibly getting infected. At the very least, you likely have a blood blister. Nevertheless, you need to see your doctor right away to make sure it's not infected and also to get a tetanus shot if you haven't had one recently.

Nail black: Any change in the color of the skin after a puncture should be assessed immediately. There are many things that this could represent from a collection of blood, decayed tissue to an infectious process. Go soon.