There are three loci responsible for the black skin trait or fibromelanosis, therefore the black skin trait is polygenic.
The gene that is sex-linked is called dermal melanin the other locus is fibromelanosis. dermal melanin is a recessive gene, so a male needs two genes to add black pigment to the skin, females only need one of the dermal melanin. The other locus is fibromelanosis. Fibromelanosis is a dominant locus. The third locus is the dominant white skin gene.
If you cross a longhorn with a silkie, all the offspring will inherit one dominant gene for white skin and the fibromelanosis locus. The females will not inherit the dermal melanin from the rooster- the females will inherit the dermal melanin inhibitor gene ( no black pigment in the legs and skin) from the father. The male offspring will inherit one dermal melanin from the hen and one dominant dermal melanin inhibitor from the father.
female offspring will be white skin, one fibromelanosis, and dermal melanin inhibitor - the fibromelanosis locus will not be enough to make black skin
male offspring will be white skin, one fibromelanosis, and one dermal melanin plus a (dominant) dermal melanin inhibitor- males will not have black skin
the only back cross that will produce black skinned offspring is with a silkie or some other black skin breed