Black East Indies Gentics Question

[Knighstar679]

I have been doing more research on the Black East Indies. I have found that their is a blue coloration as well as the straight black. I was wondering if this is a mutation or something else. Is there a chance that a normal straight black BEI will throw a Blue?

I really am curious how the genetics work in this.

 

[MagpieDucks]

Not sure how it works, but I think it would be pretty rare.