He was barely introducing Zoro to basic haki, he was not teaching him some advanced haki teachnuque only 2 swordmen in hystory were known to have lol
You realise that Mihawk was talking about making permanent black blades not just coating your sword in armament.... Do you remember the conversation? Mihawk about asks Zoro about his swords breaking and tells him if his swords were black blades they wouldn't even get chipped.
Get it? We know this is not true about actual coating swords in armament since Zoro himself says that if one guy's armament is stronger than another guy's then he will cut through him... So if Mihawk literally says making black blade means your sword can never be broken he would be lying if he simply means coating a sword in armament like you armament suggesting
So no, Mihawk has said that any sword can become a black blade snd everything else you are saying is head canon
In fact your point is just wrong to begin wth since Enma isn't a a cursed sword, Shusui and Yoru have never been stated to be cursed swords, the only cursed swords in the series are the kitetsu blades and Law's sword and they ARE NOT black blades and once again Mihawk says any sword can become a black blade