A solvent based cleaner just isn't going to cut through that. Use a mild abrasive; toothpaste or an ink eraser will work, but be careful not to remove any plating materials from the spades. Absolutely use a good non-oil based contact cleaner to follow up (not Progold). Crocus cloth is a favorite for oxidized spades, AC plug prongs, etc. It only looks like fine sandpaper, but is much finer than even #1000; available at better hardware stores.
