The White Legbars are created with a Recessive gene for white plumage. If you cross them with a non-white cock bird you won't get any offspring that are white. 0% white, 100% colored
If you back breed one of the cockerels from the first crossing to the White legbar you would get 50% white, 50% colored because the colored cockerels from the first cross would be carriers for the Recessive white gene.
The Blue Eggs are produced by birds that have a White, to Tint, to Light Cream-based colored shell with the addition of a dominant gene that add the blue coloring to the shell. If you cross a Legbar with a white egg breed, then they will still produce blue eggs (although more pale in color since the blue gene will be split and not pure breed). If you cross to a brown egg laying breeds like an Orpington or SFH the egg color that is added to the shell will mix with the brown egg shell colors producing a green, khaki, or olive-colored egg.