I cannot get my head round genetics, too scientific and complicated.
When my purebred lemon cuckoo Pekin, Pierre joined his 7 purebred ladies, 1 blue, 1 lavender, 2 lavender cuckoo, 1 Millefleur, 1 white and 1 lemon cuckoo ,I incubated the first 13 eggs. Of the 9 resulting chicks there were 3 blacks who were obvious, so that must be a dominant gene from the blue,
but sadly they all died almost immediately apart from a buff, whose mum was, I assume, my Millefleur, and older and much more hardy girl from year before last.
Don't know if this helps in your quest for a black Partridge Pekin or not!
Happy Christmas anyway!