So I went out and broke that egg open to see what happened and the ducklings beak was right underneath that dark spot. It's rear was backed up to the air pocket. So you were right about the dark spot. Question is, does the duckling being in there backwards mean certain death? If I see another dark spot forming on another egg can I try to put a hole there so it can breathe? There was a good bit of blackish green stuff in the egg too. Like the fluids were rotten already. The yolk was completely intact. Sad burial this morning. But I wonder if that happens again if there is some way to try to save it. I am guessing it would drown if you put a hole in the egg.
Oh no, I'm so sorry. :( That's exactly how my girl's egg looked when she was trying to hatch out of the wrong end as well last year. It is not a death sentence if that happens, they just sometimes need help since they are not hatching out of the air cell like they should be. My girl made it out, but only because I intervened. She had a sibling who was not so lucky. He/she was also malpositioned (they were shipped eggs, so I attribute multiple malpositions to rough shipping), but I didn't notice that one in time so it died in the egg.
But yes, if you see another one with a spot like that again, I would make a very tiny hole at that spot so the duckling can get air. Then you'll want to prop that end of the egg up a little higher than the other end so the duckling does not drown in the fluid inside the egg. With a normal hatch, the duckling will break into the air cell inside the egg, where it has enough air for about a day while it prepares for the next stage of hatching. The problem with them hatching upside down is that they don't have that air cell to break into, so they often run out of air (or drown) before they're ready to start breaking the shell to hatch out.
I wish you luck with your next egg! Is that the last one you have in your incubator? So sorry this has been such a sad experience for you so far.