we know imu cant forge a black blade like mihawk and ryuma
he is not strong enough to forge one
we will just see swordsman imu's blade being a pitch black shade due to him infusing his blade with acoa acoc and maki-omen .
reminder that haki is invisible in one piece verse so it getting a temporary pitch black shade is a clue of how to forge a weapon permanently black.
Mihawk>>Ryuma>imu
