i think i read that the mystic onyx is a cross between the Cemani and black silkie. i know someone that has a onyx and breeds his partridge Plymouth rocks with it, pretty flock of chickens.I got some of these similar beauties from Tractor Supply last year!
They were marketed as Black Diamond. (Similar to Mystic Onyx?)
They seem to be a mixed breed combo of melanistic traits. The black skin, iridescent feathers, and extra toes mostly breed true so far when between each other (at least my 1st gen.) They can give the black trait to other breeds (and also the random poofy feather top).
Most of them are a very charming personality! Smart, friendly, and very good layers of med. Brown eggs. if I could find them again I would buy more. but for now hopefully I'll get a few more chicks!