Hens are easier to melanized and Fm is only one of the genes responsible for the full phenotype. I suspect mulberry Face and other enhancers are at play
Blue/Black Ameraucana is enough to produce the results you are looking for, the good thing about Ameraucanas is that they carry the pea comb and beards that means that the Test Cross can be sold as Easter Eggers if need be without people actually thinking they are purchasing Cemani type birds
Ameraucanas would be my go to breed choice for out crossing for testing purpose, first they have the sex linked dermal enhancer gene(id+), White skin gene(W+/W+) and lay blue eggs so they will be cool looking like Rapanui or old chilean stock of Mapuche
Both the Chinese Silkie and Ayam Cemani have been bred for thousands of years for their black skin/meat/face, recent genetic studies have confirm that they share the same mutation and give it a date of around 9000 years ago(well before the chicken domestication event)
Reference: The origin and...
This Test is riddle with Flaws, not to mention that it lacks any basic genetic knowledge or even attempts to educate it's users on the fundamentals of the black skin on Fibromelanotic birds
Edit... stay put for a detailed genetic explanation on the matter...