Here's a view of the inside. If you cook them, the dark area tastes both fruity and bitter.

Two possibilities come to mind. Black Rot or Souring. You can Google both for pics and info. I'd bet on Souring.

The way to decide between the two is the condition of the plants themselves. Black Rot also shows pronounced effects on the plants while Souring doesn't.

Personally, cooking/eating of the affected areas is NOT recommended and it should be removed before cooking so it doesn't affect the flavor of the rest of the potato.

Dave

Lol, Dave! I do my best to cut out the spots, but they sometimes seem to be forming under the surface where I can't see them, sometimes not until I've taken a bite. Or I miss a bit when cutting and don't notice. I'll search for those two when I have abit more time, thanks!