I recreated a cemani look alike and got very far in about two years before I stopped. Find some rapanuis with black skin, sumatras, quetros with black skin. Now remember, a true cemani has black meat as well and this will start to happen (rapanuis bones are already quite black). There are several genes involved and it takes all of them plus some development to get the birds that way. Silkies can be used as well as other black skinned birds to enhance the effect. Concentrate first on the pigmentation, then clean up the bird to fit the phenotype.
BrianWI