This sure sounds a lot like potatoes that have been stored too long, in too cold of an environment before cooking. When potatoes are held below 41°F for too long a period, the starches convert to sugar and it changes the cooking chemistry. Instead of a nice caramelization, potatoes will often cook up overly-sweet and have unappealing, odd shades of black or gray.
Here’s a quick test to determine if your potatoes have been stored too cold at some point along the distribution chain, and how to resolve it, if so:
