If you were looking to produce sex links...

[Krys109uk]

If you put a copper black over a cuckoo marans female you'd get sex linked offspring, feather legs & dark eggs.

 

[Promiselandfarm]

But I don't have cuckoos only BR and regular marans

 

[Krys109uk]

Sorry, I got confused between you & the OP.

Copper black Marans crossed ono BR, the offspring would usually have some leg feathering. Egg colour would be variable in both crosses you suggested.