The color of the darners’ caps can change indeed and the spots may appear. Dark spots usually grow on top of the caps if we store mushrooms wrongly and they already started going bad. Unfortunately, such a sign means that darners are not safe for eating any more and they must be thrown away.
The optimal way to prevent these spots from appearing is to check raw fungi regularly while keeping them. Also, ensure that you provide the boletuses with proper conditions. When we store raw mushrooms, they must be dry, placed in the airtight container so that the air couldn’t bring odors to them.