I strongly suspect that the black chicks are crosses. How many hens do you have? I ask because there is a recessive black gene. If you have a cockerel and hen both carrying recessive black, then it you would get about 25% black chicks out of the pairing. They will come out looking like golden cuckoos. Check their leg color. If they are not yellow then it would support a not pure Legbar theory. Also as they develop watch the ear lobe color. If they get red lobes they aren't pure cream leg bars. Also watch the egg color when they start laying. If the eggs are greener than the eggs they hatched from, they also would indicate that they aren't pure. If they are pure Recessive black, then if bred back to a standard color Legbar they would bring back the standard color. I wouldn't breed them though since you don't really know what color they would be carrying under the black and could set you back in your breeding.