This is a great question! If we had a FAQ page, this would definitely be on it.
Short answer is: your Zombies need to be made by pairing your Cemani cock over the Leghorn hens. Never the reverse.
Long answer: 
Crossing a Cemani cock over Leghorn hens is the only way to go here. It has everything to do with sex linked genes. 
Leghorn cocks generally carry two copies of TWO genes that have a negative impact on fibro expression. Those genes are the Inhibitor of Dermal Melanin (Id) and they usually posses the barring (B) gene. Both of these genes reside on the Z sex chromosome. Hens have one Z chromosome and one Y chromosome. Cocks have two z chromosomes. 
When a Leghorn hen is mated, she passes one of her sex chromosomes to her offspring. If she passes her y chromosome, the offspring will be female. If she passes her Z chromosome, the offspring will be male. Her male offspring will inherit one copy of Id and one copy of B, which means that her sons will not posses dark skin. Her daughters will. Her daughters will be good Zombies (if you have paired her with a good cock).
Because the Leghorn cock has two copies of a Z chromosome, he is going to pass a Z chromosome to his offspring every single time. This means that all of his sons and daughters will inherit a copy of Id and a copy of B. All of his offspring - even if they get a copy of Fm and other melanizers from the Cemani hen - will have the fibro expression negatively impacted. The chicks will grow out to have red combs with black splotches, yellow legs with black splotches. It's generally not pretty. The plumage will be black and white barred like a Barred Rock.