I have found that the duck will push out a dead egg. Also, i read on this thread that a dead egg won't hold heat so if your dark egg is warm i would leave it with her. I almost threw out an egg that was dark but because of what i read i didn't and it hatched two days later. Hope this helps. Good luck
one of my hatches was on the 35th day, I almost threw the egg out. the next hatch was right on time. hopefully everything will go well this time for you. good luck 
