Thanks for the answer. Black sexlink is also good for me. Are you planing to have any ameraucana x rhodebar chick next year? I would like to have one of those too.Blue (as in BBS - Blue/Black/Splash) is partially dominant, so a blue roo will produce about 50% blue chicks if mated to a black hen. Lavender is recessive, so with a black hen (that is not split to lavender) all the babies will be black split to lav. That works fine for sexing BSL chicks because they look black (the recessive lavender is completely hidden). If a chick that is not black (blue/splash/lavender) hatches, they are not sexable, even if the femle parent is barred, you just won't see the effects of the barring gene unless against a black background.
So, if I make the cross I described, the BSL pullet chicks will grow up completely black, but carrying the (hidden) lavender gene. so I won't have sexable lavender chicks, just black. As with any hybrid, breeding from those hens will give tough to predict results.
I found a place online that will DNA sex chicks from their eggshell, but it costs $10 per chick, so I don't see that as feasible unless it is a high value chick..