My actual guess would be a blood spot.. See following link..
Common egg quality problems BYC
The other consideration.. if it had started developing (which I don't think so, did they sleep on the nest) would be a blood ring that goes around the outside I think it would be in the white not on the yolk.. indicates an embryo early death.
I usually feed eggs that have been left
in the incubator.. back to the animals.. I used to scramble them, sometimes now boiled and smashed.. Once I misread the candling and accidentally had an eye looking at me when I cracked the egg open.
If ambient temperature.. they should be safe for human consumption, if there were no crack allowing for bacteria to enter.. I mean.. some locations they eat what's known as balut eggs.. basically full term unhatched embryos though I am unsure of their preparation method.