Why did my potatoes turn black?
Almost always there are two reasons the potatoes turned black... exposure to air or oxygen and black spot bruising. As soon as the potatoes are cut I would try to get them into some sort of water and acid mixture, usually a gallon of water needs 1 tablespoon of concentrated lemon juice or white wine vinegar. Here are a few Dr. Potato blogs that will help explain this:
The bruising comes from storing the potatoes too cold. A pressure bruise will show up underneath the skin but the blackening may not appear until the potato is boiled or cooked in some manner. Here is a link explaining: