a potato turns black while slow cooking?

If they were ok when the went in they are going to be ok now.

Pick one out and cut it to see if it is a surface stain or has soaked in. Then have a taste test an see if you can identify the tast.

I can only think mushroom juice will colour it, or perhaps black sauce or something

Dune

There is something in the crock pot that can turn potatoes black. It's called oxygen. Certain foods will change color when heated and left exposed to air - potatoes are one of them, apples are another. They became oxidized. Anti-oxidants like various acids, lemon, citrus, tomatoes, vinegar, can prevent oxidation, but you probably have no acidic ingredients in your recipe. Another solution is to keep stirring the ingredients so that the potatoes don't sit on top and be exposed to air for very long. Crock pots tend to have this problem because people think they can just throw in whatever they want and ignore the cooking process for hours on end. They're wrong, as you just discovered.

Crock pots are easy to use, but they encourage a lot of bad cooking practices. Most vegetables lose all their flavor, color, and texture when cooked for hours on end. They do not improve with long cooking. Only tough stew meats get tenderized with low and slow cooking. So it's better to cook the meat with liquid, dry herbs and seasoning for most of the cooking time, with the only vegetables being tomatoes and aromatics like onion, garlic, and celery, which are ok to turn to mush to season the meat and liquid. Carrots, potatoes, and most other vegetables that you want to survive intact should be only added near the end of the cooking time, 30-60 minutes before serving.

The potato is fine to eat, just visually unappealing. You can cut away the black oxydized parts to get to the white starch underneath.

Soften the potatoes however you like, whether it be boiling or microwave. Then peel the skin and mash them up. Add sour cream, butter, cheese, garlic salt, pepper and mix it all together in a bowl. Then spread them out over a dish and bake in the oven at 425 for about 15-20 minutes. When they get done they will have the taste and texture of a double stuffed potatoe in half the time.

In the superstore, fruits are usually selected far too soon. Some are rocks, many are bad. Some of the fruit and vegetables are right (zucchini, onions, garlic, lettuce, greens, and a few others) so I'd have to go with vegetables.

Fruits are identified as ripened flower ovaries which produce seeds.

Are you sure those were potatoes?