Good question. Don't know much about pure breeding, but I'm guessing it's bred in from one breed to the next. Or maybe it shows up on its own when making the self black color, as it apparently takes many genes that work together.He's really neat looking!
So, since Jersey Giants now come in blue, as do Australorps, then some individuals within those breeds must already carry the blue genes.
Or, do you think they mixed in a different breed initially to start getting the blue gene in there?

Here's a page if you're interested in chicken color genetics. (not the easiest read, but it has nice illustrations

@jmns I replied before I saw your post... Try the link above, and also this: https://www.brahmacochin.org/poultry-breeding-genetics It's about brahma's but same genes for Orpington I guess.
I live in Norway.
