The black stuff can be from sporulation. I.e. the caps dropped spores from between the lamella you see under the cap. Especially if the caps were just about opened all the way (or almost). Strange that it can end up on top of caps but even the slightest air draft can sweep it up and over.
If you weren't very late and it doesn't look or smell exactly moldy fuzzy or gooey try rubbing a little off. If it's like soot/grime, like an extremely fine as good as dry dust it's probably spores. They can be purple but in huge numbers appear black. Some may have a different appearance for different strains (Albino is an example).
Don't wait too long with drying or eating after you picked them. They can be kept for a short while in the fridge but only done correctly.
Otherwise dry right away, look up how to do this, it's important to do right!!