Dick Arentz describes (in his book) a useful method for eliminating black spots with the use of Titanium White water color paint. I have found that the offending spot may be reduced to the surrounding tone with a dilute solution, or a stronger white tone can be applied and later (after drying) can be filled in with the proper color/tone of watercolor paint (my preference being Lampblack). I have managed to salvage numerous old prints which I once thought were hopelessly "plagued".