The egg color of marans really varies from breeder to breeder, hen to hen, time of laying season and such. That being said, Copper Blacks lay some of the darkest eggs, although I have seem some VERY dark Copper Blue eggs.
In terms of Color they are similar except one is black and one is a dusty gray, both with the beautiful copper highlights.
The black and copper of these birds works similarly to other blue/black mixes in that the offspring will be blue or black (in these cases with copper) or splash.
Blue Coppers are still a work in progress, like the rest of the Marans. But they are one of my favorites and on my short wish list.