How common is the fibro mutation in EEs?

[MerryFeather]

Genetics question...How common is the fibro mutation in EEs? We have a common EE roo (parents from the Wilco bin), white with caramel colored wings and just a couple of black tail feathers. We crossed him with hen that is half California White, with some Salmon Favorelle and EE. They produced an all white hen, amongst others. His offspring roos have had grey combs by 2 weeks, until turning (healthy) purple and then red at maturity. We then crossed the roo with his white hen offspring and got one jet black, black comb, black beak chick. All other chicks in the hatch (10 total) have basic mutt EE coloring. Chicks from roo+his offspring have some blue/black coloring in their beaks. Thoughts? Thanks!

 

[AmeliaBedelia]

Are the earlobes and skin black? Sometimes chicks with black down can start out all black but it usually fades.