I sliced russet potatoes in a food processor then parboiled in milk, cream, salt and pepper Then refrigerated them in an aluminum pan overnight. The next day most of them were greyish color with black outline, I had to dispose of them. I use aluminum pans when making potato salad by parboiling the potatoes and nothing ever happened. Was it the food processor, type of potato, alum pan or all the above. I never want to be in this situation again.

Good question. Whenever someone has an issue with peeled or cut potatoes turning gray it usually can be traced back to exposure to air or oxidation. It can also be due to bruising, which could have happened by using the Cuisinart to slice the potatoes. The aluminum pan is also a pretty good clue as metal will also react with the potatoes and the acidity of the milk. So three possible choices.

My bet is on the exposure to air. Whenever I peel and cut potatoes I immediately put them in a solution of water and some acidity from concentrated lemon juice (1 tablespoon per gallon of water) or white wine vinegar or the trick food stylists use, Ball® Fresh Fruit Protector which can prevent browning of fresh cut produce. A 5-ounce jar typically runs $6-7.

This method, adding acidity to the water works well for potatoes, apples, bananas, etc. to prevent discoloring.  You may also want to get a set of clear glass baking dishes instead of using the aluminum in the future.