Just because a hen lays blue eggs does not mean she has two copies of the blue egg shell gene. It means she has at least ONE. It's a dominant gene that only requires one copy to express. She may also be 'carrying' a gene for white egg shells. If that's the case, then she may produce 50% green layers and 50% brown layers. This is assuming that all the chicks inherit the genes for the brown coating, as well. There is a small chance that a few won't get those brown coating genes at all, and thus, they will lay blue or white.