Fair enough, but this one is more up to Oda's discretion. Might be too much effort for him to do so on black pants when he can get the same result from a statement (haki was used). I think this fandom puts more stock into hardening than Oda himself. It's mostly for the visual effect, which is why no one pre skip could be seen using it.
If it's a given, via statement or common sense Oda might not put enough effort to even sweat the difference, it's implied Vergo was using full body haki, we know his pants were still white doesn't really change much.
Pica's whole body was black but not the straps around him. Sometimes it goes under clothes.
So the Judge panel remains inconclusive.
Like i said, it's nothing serious, if the message goes through loud and clear. In the judge scene we know he used haki, and that was probably good enough for Oda. Hardening is mostly visual effect so we as readers know haki is being used, if the same result is garnered via statement it's not a big deal, atleast it doesn't seem so for Oda.