Black spots in potatoes are mostly attributed to internal bruises or the result of sugar concentrations brought on by any of several pre or post-harvest conditions and are generally harmless. However, the black spots could also be an early concentration of decay. Again, probably not entirely harmless following the cooking process but I recommend not taking any chances and cutting and discarding any such obvious discoloration. Idaho potatoes are very economical so whatever waste exists in skinning or slight defects is a minimal sacrifice– while the nutritional and heart-healthy attributes are substantial.