Jane, after thinking about the shell color genes..... When you cross chickens with blue genes with chickens with a brown coating gene, the resulting chickens only get one blue gene for their shell color. So I am thinking if you cross that chicken with a leghorn you may get a blue egg, but you may not. IF the brown coating gene gets lost in the cross you would get a blue egg. But if the white gene dilutes the blue gene and the brown gene, you will get a pretty mint green egg.
I am by no means an expert on egg shell color genes, but, I think I understand well enough to explain what you wanted to know. If you find out my information is incorrect, please let me know!