All advanced haki users don't use black hardening, they just use regular armament haki. And I think there's a valid reason for it, whether Oda does it consciously or not, or whether he'll ever confirm my suspicion or not, but I believe it's a stamina conserving move. Every top tier who encountered gear 4th, which infamously uses ton shit of black hardening, have all pointed it out that it's super inefficient, retarded and that you're gonna gas out super fast, hence the time limit where Luffy can't use haki for 10 minutes because he wasted all his haki–and as far as we know he hasn't fixed it yet, we just don't see it anymore because he rarely uses gear 4th long enough. Same can be said about all other top tiers, none of them used black hardening, whether the Admirals or the Yonko. The only time we've seen them use it in a fight is with Aokiji vs Garp, but even in that fight it was a very limited use. We usually ask how can characters fight for multiple days straight when they gass out so fast, well if it's two veteran top tiers who are not rushing, they'll try to conserve their energy as much as possible. Same can be said about CoC, although to a limited degree. Gaban has already pointed out that Luffy doesn't use his CoC efficient enough, and Zoro was just wasting his CoC by not even being able to control it at all.