I am going to make a couple of assumptions, so correct me if this isn t what happened
The peeled potatoes were raw. When slicing they were exposed to air (which will cause them to quickly turn dark). One method to avoid this is to peel the potatoes, place in water covering the potatoes, remove and slice on a cutting board with a knife that is steel (some blades have iron in them and till will turn the potato dark). Then place back in water, drain and dry before cooking on a shallow aluminum tray. The cooked potatoes were allowed to cool to room temperature, then removed from the tray and placed in a plastic container in the refrigerator for the next day. If the potatoes were left on the tray the potatoes could still turn color from the aluminum pan. If the cooked potatoes were not left out overnight, unrefrigerated then they will be okay to eat. They will be pretty unattractive to the eater but will taste fine. If they were left out, then because of the dairy (butter) they would not be safe to eat.
Found this info. on the Internet on several sites: You can make frozen French fried potatoes at home. The mealy type potato is best. Use mature potatoes you have stored for at least 30 days. Potatoes with a higher sugar content will brown excessively and will be less attractive. To make frozen French fries, pare and cut potatoes lengthwise into 3/8 inch strips. Rinse in cold water to remove surface starch. Dry thoroughly on a towel. Fry strips in deep fat or oil at 360 degrees about 4 minutes or until cooked, but not browned. Remove from fat and drain on absorbent paper. Do not overload the French fry basket. If you do, the fat temperature will drop and the potatoes will absorb more fat. Use just enough potatoes to cover the bottom of the basket. Cool French fried strips to room temperature and pack in cartons or other airtight containers. Seal, freeze and store at 0 degrees or lower. Homemade French fries have a short storage life, so use them within 2 to 3 months. When you use them, fry in deep fat without thawing.
For skillet crisping, brown the French fries in one tablespoon of oil. A 475 F oven may be used to finish cooking and browning. For hashed browned potatoes, prepare as you would to serve. Brown only to the brown-and-serve stage, cool and package for freezing. Store frozen for one to 2 months. When you want to use them, finish cooking and browning as for regular preparation. Prepare mashed potatoes as for serving. Cool, then pack in airtight containers and freeze. Slice when ready to use and fry without thawing. You can also make mashed potatoes into mounds. Freeze on a tray and then package. Store for one month. To use, take out as many mounds as you need to fry or put on top of a casserole. They can also be reheated in the microwave. To stuff baked potatoes, remove the cooked potato from the skin, mash it and add seasonings. Return to the skin, piling lightly. Wrap each potato in airtight packaging and freeze. When ready to use, remove wrap and put directly into a 425 oven.
Bake for 30 minutes. After 15 to 20 minutes in the oven, top each potato with grated cheese. cooked, such as mashed potato patties, baked stuffed potatoes, or french fries. soups and stews do not freeze well. Upon thawing and reheating, they tend to disintegrate, especially if the potatoes are whole or cut in large cubes. If you are preparing a combination dish for freezing, omit the potatoes and add them instead during the thawing/reheating step before serving. Make and partially cook fries, homefries, and whatever other style first. Then freeze in portions (big or small – size doesn\’t matter). The real trick to freezing potatoes is cooking or partially cooking them first. Raw or new potatoes just don\’t freeze well. I think it has something to do with their water content -they turn mushy! Though it takes a little time to cook before freezing, you make up the difference on the other end, and, in my experience, don\’t lose any flavor. Certainly better and less expensive than commercial products.