I think I am experiencing the same situation with patch-like areas difficult to blend away with sp or polish but I wondered if anyone has ever posted a picture of pitting ....
The dark spots you see could very likely be pitting, Check out this picture below. The dark line running from toe to heal was a long line of pits. At first I thought it was gunk or tarnish, but after I sanded off the dark tarnish I was able to see the pitting.