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A single copy of O (the blue eggshell gene) will cause a female to lay eggs with blue eggshells. The cream legbar can be assumed to be pure for O (meaning htat it has two copies) so ALL its offspring will inherit a copy of the gene. Yes, if the legbar is crossed with his daugters, they may have two copies of the gene, which may deepen the shade of the blue eggshell.
Green eggs are formed when the hen deposits a brown coating on the outside of the eggshell. There are many genes involved in this, not just one, and can vary from barely tinted to a very dark-chocolate shade. A hen whose eggshell is white, and that also deposits a brown coating will lay a brown egg. Those whose eggshell is blue, and deposit a brown coating lay a green egg. I could speculate that a very lightly tinted egg, which often looks pinkish, would look more blue than green if the eggshell is blue. But is the hue of the coating is more brown, the egg would look more like green.
Depending on the genes for the brown coating carried by the Isa Browns and the black sex-links blue eggs are possible. Isa Browns should lay brown eggs, but in googling, I came up with a number of instances where they lay white. Black sex-link is a generic term; depending on the particular breeds crossed, it would be possible to have ones who lay white eggs. Probably not usual, but possible.
Questions to ask the seller are whether the hens are first, second or subsequent generations of this cross. Also, ask for photos of the blue and green eggs. Are the eggs you would be getting going to be white, brown, blue or green? If the eggs are laid by hens who are not crossed with the legbar, the colour of the egg will tell you at last half of the genes they are likely to inherit for white vs brown coating. If they already have the blue egg gene, the egg colour will tell you the same--just lookk at it as blue vs green raher than white vs brown.
