I cut potatoes into French fry slices and left them in the refrigerator for a few days. They are partly black in color, are they safe to eat?

The raw potatoes turn gray from exposure to air. When turning black, that means they have been exposed way too long. I would toss and start over again. Next time, cut the potatoes and put in water with a little acidity (1 tablespoon of white wine vinegar or concentrated lemon juice per gallon) and this will help keep the submerged potato strips from discoloring when you refrigerate them until ready to drain and fry.