Your Cochin is a Black bird with Mottling. The genes responsible for putting the melanin (black) into the feathers also puts it into the beak,eyes & shanks. Another gene, Inhibitor of Dermal Melanin, removes it from these areas, however it is sex linked meaning that the males is homozygous ( has two copies) and thus a greater effect than that of the hemizygous hen that can only have copy.
Hormones also play an part, hens being naturally darker than cocks, look at the undercolour to see what I mean. If your Cochin hen has yellow foot pads as shown she has the correct skin/shank colour.
To get exhibition yellow shanked black hens in a single breeding pen takes a lot of breeding knowledge, large hatches, and plenty of space and time, not to mention money.
However most top exhibition breeders of yellow legged black breeds have separate breeding lines for producing exhibition males, and exhibition females.
David