I noticed the darker splotches on "Flips'" egg when he was real close to needing help getting out. The membrane was drying and pulling away from the shell. I found the color of the membrane was a great indicator of its dryness/toughness - white first, changing to tan, then brown and then nearly black. The darker the color, the tougher the membrane and I know there was no way our little guy was going to be able to tear the membrane past the dark brown point. He also was visible in his proper air sac for a period (about 24 hours after we noticed the external pip), and we could hear him cheeping, but ultimately he still went back to the original pip hole.
The coconut oil was extremely helpful not only in seeing if there are still blood vessels, but also in helping the shell come free when we had to assist. Allowing enough time for the re-absorption is critical and we struggled to be patient and not just jump in and start removing shell, but I will defer to the experts on here as to when to step in. Sending best wishes for you and your little one!