MY SISTER HAS WARTS

You can google "warts turned black" and see lots of articles about warts. They are very common.
The black is just old blood. It means nothing.

My daughter went through a stage where she had warts that come coming back. Warts are very stubborn things. The best way to get rid of them is anything that irrates them. So actually stepping on them and turning them black could have caused killing them - although I wouldn't normally suggest that as she could get hurt.

One of these stubborn warts (was on her thumb) we went to the doctors - her doctor wasn't available so a nurse treated the wart - my doctor's office is to freeze them. Well when the nurse did this, she wasn't as careful as the doctor and did not cover the skin surrounding the wart. The next day her thumb turned black and blue and a bubble formed around the wart - I brought her back to the doctors where they said the freezing irrated the skin around the wart causing a blister. The bad thing - it hurt to write for a couple of days - the good thing - the blister lifted the wart, killing it.