First time hatching duck eggs
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Can you get a quick pic of the eggs? If they are turning black (that you can see from the outside), they are rotting and need to be tossed. If you take a little sniff of those eggs you will probably smell that they are not good.
Have you candled any of these eggs during incubation? I would suggest going out at night when it's dark and candle them all real quick to see if any are still good and if you can see the embryos moving around inside.
Ok, well a picture would help at this point if you're able to post one. ;) Eggs do not normally turn black during incubation unless they are rotting inside.
Cayuga eggs can have a dark grey or even black "bloom" on the outside of the shells when laid, but they will not turn darker or black as the eggs get older. That is not normal. (I have Cayugas myself.)