For whatever it may be worth, Dr Okimoto said in an older thread on The Coop that the best exhibition black silkies were probably birchen based. He isn't on there any more, so I can't ask "why?". I'll see if I can find a link to that thread, or at least others' commentary on it.Ok now look a little closer at that list of black alleles. Where do most blue birds have the dark color? the head, hackles, shoulder, saddle and breast feathers
Here's a pretty good example of a blue I used to have.
Now look at that and go back and reread the description for the birchen base. E^R (birchen) - needs help from black extenders to make a black bird . Now go back and think about those poor blacks from blues.... Get a birchen based black with not enough black extenders and I'm sure you would have an easy route for those poor blacks. Makes for nice looking blues, but the blacks resulting from it are not going to be that gorgeous jet black all the time.