I know a breeder that was crossing partridge and blacks to improve type and I know someone that ended up getting some of the product birds of a couple of the generations later. They showed a little red leaking through on the neck and wings, and saddles, but after careful breeding and hard culling he eventually had a really nice line of pure blacks bred out without and red leaking through. He did well with them while he was still showing, but I think most of the birds are gone now, since he's been out of birds for at least a couple of years now. But they were all the direct product of breeding partridge to black to improve type, but he had nice blacks that didn't show any of the influence of red after two years of breeding them himself, (with the original stock probably being two generations from the original cross.) It just takes CULLING (meaning simply getting rid of, by what means, it doesn't really matter.) And careful breeding.