It's for sure possible, My BCM's are not hatchery birds and lay a very very dark chocolate colored egg. They have been my only experience with the breed. I said Black star because of the look of the bird and the description of the eggs. My Black Stars lay huge eggs that are brown, my marans eggs...
I don't believe the black hen is a black copper marans.. It has no feathers on its legs, and most BCM I've seen don't have color that low on her Breast. I'm going with Black Star ( or black sex-link) what color are her eggs?