Have you hatched these before? I am also hatching Marans eggs. I am worried about what I've heard about them being hard to hatch.
I've had good hatch rates with them. You just have to be patient. In my experience, they are always the last ones hatching and it is a slow process. Case in point today, 6 of my 8 chicks have hatched, and the two that haven't are the two BCMs. One has been pipped for over 12 hours. One just finally made a tiny pip 8 hours after all the others had pipped. I'm just reminding myself that this always happens and to relax about it.