So sorry to hear the plans for a casserole dinner were ruined. What an expensive result.

Potatoes turning black – good question and here is the short answer… raw potatoes can’t be frozen. There is an enzyme reaction that has to take place to keep the potatoes from turning black. In the future, you’ll need to cut the potatoes, placing them into water with a little lemon juice or white wine vinegar (something acidic) and then you need to blanch them by heating in water till mostly done. Then drain and layer your casserole as usual.

You hit upon the scientist’s discovery of how to do frozen French fries. Ray Dunlap of Simplot found that if he partially cooked the potatoes they could be air dried and frozen for frying later. Thus, McDonald’s switched over to the crispy shoestring potatoes they still use today.