Sure, but Imu does show that Oda isn’t afraid to draw a blade in the black-and-white manga as black, even if it’s not actually a Kokuto. Obviously if GB’s blade isn’t a Kokuto, the reason why it’s drawn black would be something other than Imu’s magic. It could be a grass sword, wooden sword, or Oda just thought it looked better with the rest of Aramaki’s design if he drew it black (same reason why he drew canonically blonde Queen with pitch black hair in the manga).
Tbh, if Aramaki doesn’t even show off any swordsmanship in his next major encounter, I’m just gonna assume that he either got the Kokuto from someone else, or it’s not really a Kokuto. Someone who forged a Kokuto should obviously have swordsmanship be a core part of their fighting style