Ok, is that the fat end of the egg (which has the air cell in it)? I can't tell from the angle of the pic. I'm guessing it is since you said you can shine the light through it. Very hard to see the dark spot as well, but I can just make it out. It could be trying to pip there. When you candle it can you see part of the shadow of the duckling sticking up in that open space by this dark spot? That would be his bill and indicate he has internally pipped and is working his way out. Has this spot gotten darker since you first noticed it? Ducklings do rest alot during the hatching process (typically 12-24 hours between each step of hatching), so it's still possible it is alive in there and was just resting when you candled. What day of incubation is it on? With call ducks I believe they typically hatch around day 26, and not day 28 like most other ducks, but that is just a ballpark figure.
At any rate, I would candle it again and see if you see any movement or any shadows in that open space where this dark area is. You can also try peeping to the egg and then tap very very gently on the shell right there, or scratch gently with a finger nail and then hold it up to your ear. They will usually respond to that by pecking, scratching, or peeping from inside, which will tell you it's still alive and well in there.