When I hatch dark eggs, they do better at a lower humidity--30 to 35%.
It might be better for all of the hatch to split the differences, say 40 to 45% and the around 60% and lockdown.
It gets harder when hatching different breeds for sure.
This is from a Marans breeder:
I'm going to start my lockdown incubator today and put the one Marans egg in it without moisture see what happens