Black crossed to Splash will give you 100% blue offspring.
Black Copper crossed to a Blue will give you 50% black and 50% blue offspring.
To get any Splash colored offspring the Splash must be bred back to a Blue or another Splash.
You cannot get Black, Blue and Splash from Cuckoo to Cuckoo matings, unless one of the Cuckoo's that is being used is a Blue Cuckoo or Splash Cuckoo.
The solid female offspring from the BCM roo crossed to the Cuckoo female can and very possibly will show copper or red leakage in the hackle area as it matures. This amount of leakage will also depend on whether or not the Splash bird is from Copper parents or if it is truly from solid Black, Blue, Splash genetics and does not carry copper. Solid BBS genetics in Marans is extremely rare so I would venture that the Splash girl probably has copper genetics.
The same red leakage (will possibly look gold) may show also in the males Cuckoo birds produced from the BCM male over the Cuckoo female as they mature. The Cuckoo males will show leakage of red in the hackle, wing bow and saddle area.
Blue is a diluting gene and when a bird gets one copy of the blue gene it does not really change or alter any other colors on the birds except for what should have normally been black.
2 copies of the blue gene gives you a Splash bird and the 2 copies of blue together dilute even further, giving us the Splash or white appearance. The leaky blue gene allows random flecks and feathers to leak darker or lighter blue feathers and also will allow leakage of some copper. However, not all birds will leak or show copper, but this does not mean that copper is not there just because we cannot see it.
Hope this helps and if I have misdirected you that someone comes by and corrects me.
Also before the Marans police come by (*wink*), it's always Marans with an "s" regardless if you have 1 Marans or 20 Marans.
Good luck with them and enjoy them. They are a lovely breed of chickens. My favorites!
