I think I have the best explanation on why Mihawk and Ryuma have a black blades while guys like Roger, Whitebeard, Shanks, Oden, etc. don’t
So y’all know how there’s only two Kokuto users so far in the series? Both Mihawk and Ryuma devoted themselves in the way of swordsmanship. Their goals is to become the strongest swordsmen of their era. While Roger, WB, Shanks, Oden never wants to become the strongest swordsman of their era in the first place. No one have the ambition of becoming the World’s Strongest Swordsman bar Zoro, Mihawk and Ryuma. In order to make a black blade, your goal needs to become the strongest swordsman of your own era. It’s not about Haki, it’s about your ambition.