can you Rephrase your question ?
a CL x BCM will produce all of the chicks will carry the blue egg shell gene.... now if you cross those F1s back to BCM your chances are 50% of them will carry the blue egg shell gene, the issue is that you will not be able to know which hens carry it untill it start laying, and forget about the males, there is no clue as if they carry it or not, unless you do a cross mating, but can you imagine cross mating 5 of those males to see if they carry the blue egg shell? you would have to allow all of their doughters to lay to see if they carry the blue egg shell gene... thats why Ameraucanas are a sure way, or at least 90% of the time