What kind of variety of Marans? Who's the roo and who's the hen?
I hatched out a bunch of chicks from a BCM roo and some DARK egged Wellies. They were all black at hatch, and then have gotten varying degrees of brown on them as they age. I don't have any more as the one I kept was a roo. They almost all had feathered legs. My gut feeling is they will feather out like a "mossy" BCM, but should lay a nice dark egg, provided both parents were from dark egg lines.
I've always thought it would be fun to use a Wellie roo over a Cuckoo Marans hen to make dark egged sex links!