I don't remove eggs after a week and a half (talking chicken) unless they smell or leak. (Unless it's no brainer, obvious.) That late in the game, it's too hard to tell. I'd leve them in until the end. Now, I will say, I am not a duck hatcher, but I have seen many posts on here where new hatchers have asked about dark spots in the egg, or black spots in the egg so I will refer to the duck hatchers to answer what goes on when this happen cause even though I don't get into those posts, I see it frequently.