More importantly, the Blue gene is needed to shift the eumelanin markings from black to blue. Blue is a co-dominant gene which is not sex-linked.
BlBl = splash-laced (lacing a very pale shade of blue, sometimes resulting in some feathers being blue and some white - not showable)
Blbl = Blue-laced (correct blue lacing on a red groundcolor)
blbl = Black-laced (black lacing on a red ground color -- basically a gold lace, though the ground color is more red than the preferred)
As a co-dominant gene, blue-lacing does not breed true. That means if you breed two blues (Blbl) you get 1/4 black, 1/2 blue and 1/4 splash. This is the preferred breeding of those working to develop the color -- resulting blacks and splashes are culled.
If you are trying to raise chicks to sell and want to ensure that all the chicks will be blues, then breed black-laced (blbl) to splash-laced (BlBl). All chicks from this cross will be blue-laced (Blbl).
Breeding blue-laced (Blbl) to splash-laced (BlBl) gives half blue-laced and half splash-laced.
Breeding blue-laced (Blbl) to black-laced (blbl) gives half blue-laced and half black-laced.