If you breed the other way around - meaning Blue Marans cockerel x Legbar (barred) hens you will produce auto-sexing chicks, essentially Black Sex Link chickens. Boys will be barred, girls will be black in that cross. Breeding Barred Cockerels to non-barred hens produces both barred and...
The main advantage with using pea comb blue egg layers for these kinds of crosses is to track the inheritance of the blue egg gene when you get to F2+ generations. If you use single comb blue egg layers you can really only breed the initial cross because there is no way to tell if the F2...