The black spots are where the potato oxidized when exposed to air. Sometimes this is exaggerated by the metal of the fork or knife, especially older ones where the material had some iron in it.

Lesson learned; don’t pierce the potatoes too far in advance.

One trick once pierced, is to dip the potato in a solution of one gallon of water to 1 tablespoon of concentrated lemon juice or white wine vinegar, something acidic to help the potato from darkening. Most times (unless preparing potatoes for baking in the microwave) you can skip the piercing with a fork. Lots of restaurants don’t do the extra step and rarely have a potato burst while cooking.