The gene for blue eggs is the same across all chickens. It's all the same mechanism, oocyan in the shells, and it all stems from the same chickens. Cream legbars lay blue, ameraucanas lay blue, so the resulting egglayer offspring SHOULD be blue, assuming they're both O/O. (Again, variations happen especially if they've been crossed out.) The depth of color may vary between birds. Depth of color in blue egg layers is not well understood and controlled by a complex series of genes that we just don't know enough about. It's best to treat depth of color like you do depth of color in browns and just breed for the best color by phenotype instead of genotype IMO.
