my black hen is from Toni Maries German Scubert line, and just got a black roo from Cy Hyde (he has some color to his hackel and saddle though )
All these I have are bantams though, There should be pics of all of them in the earlier pages of this thread.
I was thinking you said they were bantam, but that was the mottled one , not you.
Dutch would still work though, you'd just have to breed back up for size, once you got a true solid black of any size, you can breed it with pretty much any of your standards to gain back the size, yet keep phoenix qualities and not tarnish the color.
Ducth is what most all breeders use when creating new phoenix colors, but mainly in bantams size, will have to check to see what most use for standard size. Several have used leghorns I know, but not sure what all colors are available in them to work with.
regardless of breed, For best black coloring, use solid colored birds, white, blue, lavender, etc, not the duckwings as you will end up with a lot of leakage with those. And the closest to type as possible, white ear lobes, slate legs, single comb, tail really doesnt matter, as it will take several generations to get that back right anyway.
Cy told me he was on year 8 with these lavenders I got from him, and they still arent perfect.
Cy's lavenders, I will be breeding over some others to increase my black population and blood lines this spring too. They will be crossed to his whites I got, this will give blacks the first year, then when I back breed them, it should make those lavender tails really pop, as his white is of onagadori decent, that's what he recommended I do anyway for both colors.