Most of my duck eggs have formed some discoloration at the fat end as well. I wouldn't worry about it.
The only reason I helped the last silkie is because it was the lone egg still not PIP'd and past it's hatch date by a couple days. I could hear it pecking, but feared something may be wrong. I made the tiniest hole in the fat end and chipped the shell till I could see it's beak...and it's crossed. Perhaps that's why it hadn't PIP'd. All it's blood vessels had dried up, so I zip'd part way for it and it seemed really tired. I zipped a bit more later on and it was able to slide out on it's own when ready. It seemed exhausted for a couple hours and is starting to dry and perk up now. But, my point is I only assisted because there seemed to be an issue. (past hatch date, last egg unPIP'd etc). In nature this chicky may haave passed away, being too weak to emerge due to the crossed beak...and some people would say to not assist and let it pass...I don't think there's a right or wrong there. I don;t regret helping it, and will care for it as long as it needs me to, unless it seems to be in pain later on.
