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All I can relate is my own experience, which is that my Marans eggs make it to hatching time with no problem, and then I lose close to half of them during the hatching process. When I open eggs after a hatch, without fail every Marans egg that didn't hatch will have a perfectly, fully developed, dead chick inside. It's like they just can't get out. I've heard various theories about this, from the shells being tougher, to lack of evaporation due to less-porous dark shells, to the size of the chicks relative to the eggs. I have no idea what it is, but I do know that it's a real issue. Hatched side-by-side with other breeds, the difference is notable.
I have incubated my own eggs and eggs from other folks, and one other anecdotal observation I'll make is this: If I can see into the shell at ALL during candling, that egg has a much better chance of becoming a chick, at least at my house, than does the super-dark, highly glossy egg that I can't candle.