hi Chickee .if you have access to a dark Black copper eggs Especially from Bev s stock. go and get them and recreate a proper Golden cuckoo Marans able to lay a dark egg. Because if there is a dark egg than is no point just a Golden cuckoo variety.
you all ready have a Golden cuckoo Rooster So you just need 2/3 Black copper hens and few years crossing back word and forward and ET Voila you will have a nice proper GCM your line you don t have to add any thing to t.
that what I think.
I ll help you with a genetic side of t and witch F1 birds you choose from .I m doing t my Self is Fun but if you do t properly will pay you. you ll have people want to buy. and they are worthy to sale as Marans.