But that means Shanks can replicate it with other weapons as well, be it axe or glaive. I think Roger uses CoA in such a way too. That's why neither his or Whitebeard's blades are black. The sword becomes black through perfect Haki flow through the blade for cutting purposes (though obviously your CoA needs to be very strong too), while Roger and Whitebeard just let the Haki overflow and hit you with a Haki barrier.