Its not really an error. They have nothing to base it on, but need to go with a color. Think of it more like a placeholder until Oda canonized the actual color.
The color of Mihawks eyes was retroactively changed when Oda canonized them to be red over 20 years later.
I am really starting to wonder why you all feel so strongly about this? Why do you insist?
Nothing good can come of it.
Every possible scenario ends up with an L for Admirals.
If Greenbull actually has a legit black blade, making him a swordsmen, then he is automatically under Mihawk and inevitably under Zoro too. An L.
If Greenbull stole the blade, then he is a fraud. An L.
If Greenbulls blade is not a black blade you will look like utter tools. A fucking huge L. <--- this here is what will happen