I just hatched some Rhode Island Red crossed with Black Copper Maran and got 6 hens out of 8. They only 4 weeks old.
I hatched some eggs from my Rhode island reds hens this past summer that were with a Black Copper Marans roo and the pullets came out very pretty and are now laying daily at 6 months. And to my surprise their eggs are as dark as my full BCM hens. I'll definitely do some more of this cross