Just came across this thread. I want to state up front I have never tried to hatch a Marans. However, I have read that they need a different humidity because of the shell porosity. I believe you need to increase the humidity when hatching Marans eggs. Certainaly during lock down. I have read that the extremely dark eggs have a higher porosity and too low a humidity can cause shrink wrapping of chicks.
Please take this for what it is worth, as I say I have never hatched a Marans, only read this on other BYC posts.
Laney
Hoping for your little chicks.