Both BlackLeg and Pirate hunter are outdated. Both "Dark King" and "Mountain Eater" were not titles given to them at the beginning of their journey. These titles they earned when they became top tiers, similar to "Pirate King" Gold Roger. It makes no sense for Sanji and Zoro to have the same title when even Luffy's title changed to "Yonkou Luffy" and soon "Pirate King" Luffy or "God Nika" or whatever.
People say BlackLeg is to mirror Zeff, but honestly who was Zeff? Sanji's gonna be Roger's Equal, Zeff was a New world veteran. They shouldn't have equal titles when they are leagues apart.
Pirate Hunter doesn't even make sense anymore. Is that supposed to mirror "Marine Hunter" Mihawk? We'll even so, Mihawk got a new title as WSS, so it makes sense Zoro should get the same.