You're right! Blue is an incompletely dominant gene that dilutes black. No blue gene, your bird is black (assuming he has the genes for black). One blue gene fades the black to blue, and two blue genes fade it further to splash. So your hens will give the blue gene to half her offspring. A black rooster doesn't have the blue gene, so he doesn't contribute. Half your chicks will be blue, half will be black, on average. A blue rooster will contribute the same as the blue hens. Half of your chicks will get one blue gene, a fourth of them will get two of them, and the other fourth won't get any.