I prepped my potatoes last night by slicing them, added colored peppers, mushrooms and butter. I then covered them with foil and refrigerated overnight. I’m currently baking them now and they are turning black. Are they safe to eat?

If the potatoes were slightly gray or brownish, I’d go ahead and eat them. If they are turning black, it’s time to toss. What may have caused this is that the potatoes were bruised and this didn’t show up until cooked or more likely, that exposure to the metal pan, foil or air caused the potatoes to darken prematurely.