This very dark partridge girl is probably the mother of the Darker chick. She's just extra Melanized, not actually black.Mine for comparison. Will include Male of the Dark/Melanized partridge.
I did have Australorp in my bloodline at one point, so suspecting the Charcoal Melanizer is what I'm...
Just keep breeding from what you have, & you may end up with a good partridge.
Partridge is a recessive, so it could be hidden within some of your birds, mixing it up, with reds you have could bring it out.
I use an Incuview, Incubator. Holds 27 large chicken eggs, or more depending on size. Can incubate egg sizes, from Quail, Chicken, Pheasant, Duck, Turkey, & Goose.
I'm currently getting my incubator ready for hatching eggs. Doing a Chameleon hatch(Chicken experiment, not the lizard), & a couple to hatch for a friend.