here's more proof if you are concerned. In this panel, Vergo is clearly using full body CoA, yet we do not see hardening on his gloves or his legs.
Its because he is using hardening on his body and not his clothes. Hence if Sanji, whose legs are almost always covered in pants, used black hardening, there would be no visual indication that he was using it.
However, given that he can block cutting attacks with his legs, something that fishman island Luffy was unable to do, he most likely has black hardening.