I bred Black Copper Marans for many years. When I started with the breed I came across comments from people on-line saying that the egg color was sex-linked and came from the the cockerel. I did my own test mates and proved that to be wrong. You get just as dark eggs from Marans Crosses using a Marans Hen as you do using a Marans cockerel (assuming that both the cockerel and hen are equally prepotent for egg color).
With the Hen you know what color eggs she lays. We found that about one out of 8 Marans hens would lay a significantly darker egg than other hens from the same breeding. Those were the hens that you need to breed to maintain good egg color in your Marans line. Breeding the average color every years leads to the average getting lighter and lighter every year. As with the hens, about one in eight cockerels will have the genes for the darker egg color. The problem is you don't know which male it is without testing mating it to a hen of know egg color and doing a progeny test. So...breeding a Marans Hen to a cockerel of another breed is actually a better way to go than breeding a cockerel to another breed because you know what egg color the hen is producing and with the cockerel it is either a 1 in 8 guess or requires two years to progeny test. If you have a cock that had already been through the two year progeny tested and proven to be the one in eight cockerels with the darker gene potential then you can use him with equal successes to a hen with darker egg color.
As for the color, using the Legbar male will produce 100% offspring that are barred (male and female) as show with the hen above. Using a legbar hen will produce 100% barred males and 100% black mossy hens. The advantage of that is you can color sex the chicks at day old. The males will have a white dot on their head and the females will be solid black.