With a Black roo on buff hens, you will most like not be able to sex the chicks by the color of their leakage. It would ultimatly depend on if the Black roo is silver based or gold based. It would most likely be silver and then if so, your pullets would have silver leakage, but the leakage in the males would be similar with some light hints of gold leakage too but that wouldn't be apparent until the bird has feathered out quite a bit and at that point, the physical sex differnces like comb size would be showing. Its the same concept and reason why Tim (tadkerson) says not to use Black copper roos in a red sexlink cross because the color difference wouldn't show up til later. However if the black roo was gold based then both hens and roosters would be black and buff like the birds already pictured in this thread.
However though, the reversed cross of a buff roo on a silver based black hen, like Cowgirl71 said, would be your best chance of producing sexlinked offspring and being able to see the difference relatively early. You'd get females with buff leakage like the one pictured and males with silverish leakage.