The only characters that possess, possessed or will posses black blades are those who dedicated their life to the way of the sword; Zoro, Mihawk and Ryuma (only ones we know). It's clear that Oda designed Fuji to be the Swordsmen Admiral, he's never initiated an attack without channeling it through his blade. In each Admirals introduction we get to see what they are about, with GB it's his plants which he used to fight against two top commanders, but with Fuji he is shown using his blade from the moment we meet him. It makes very little sense to have someone who occasionally uses his sword as a secondary tool beside his DF be a superior swordsmen to the guy who is the most textbook swordsmen character out of all the marines as well as relies on it constantly in combat.