Don't know if anyone has said this but imo i believe that only those with COC can use the most advanced types of haki. The only people shown thus far using them Future sight, penetration haki ect have CoC. There has to be a reason why everyone at marine ford freaked dick and said it was a terrifying ability when luffy used it. I mean as if high ups at marineford would care if all it does is knockout folder.
On to the black blade.
I think like it says in the panel when zoro found out about permanent black blade it is done through countless battles of using the flowing Coa.
My theory is that everytime you haki up the blade and go through battle little bit of thar haki stays in until eventually it becomes a blackblade. So yeah it doesn't require penetration haki i think it just requires countless battle like it was stated in manga by Oda.
Even if you don't agree you know it makes sense. I could be wrong and the penetration destruction technique and permanent black blade could be the same but then ask yourself why the fuck can't old hyogoro who said coa is a specialty in wano and obviously has trained alot lot lot of years could never reach the penetration technique that luffy can do after like a week of training I'll tell ya COC MATEEEEE
On to the black blade.
I think like it says in the panel when zoro found out about permanent black blade it is done through countless battles of using the flowing Coa.
My theory is that everytime you haki up the blade and go through battle little bit of thar haki stays in until eventually it becomes a blackblade. So yeah it doesn't require penetration haki i think it just requires countless battle like it was stated in manga by Oda.
Even if you don't agree you know it makes sense. I could be wrong and the penetration destruction technique and permanent black blade could be the same but then ask yourself why the fuck can't old hyogoro who said coa is a specialty in wano and obviously has trained alot lot lot of years could never reach the penetration technique that luffy can do after like a week of training I'll tell ya COC MATEEEEE