SOMEBODY would have reacted to Greenbull and his supposed black blade. Its like if Luffy rolled up to Elbaf where everybody worships Nika/Joyboy and somehow nobody there notices he has a Strawhat.
This is a non reason. Apparently Queen could be Franky's father, if one of the Strawhats dad was in the same room as him, got defeated, and they didn't interact, you think Oda cares for a blade?
Also the Morley reason is ass too, Morley was a massive construct in MJ. If Greenbull used his blade on him Morley's body is breaking apart and falling on the CDs below them, same thing that happened when Zoro cut Pica. Cutting a large construct doesn't magically stop it from causing damage on the way down. Restraining Morley makes more sense to try first.
There is only 1 legit reason for believing Aramaki doesn't have a black blade, and that's the fact he has a logia devil fruit. This means he can't be dedicated to just his blade, and with a blade so rare it just seems odd that absolute purists like Oden never achieved or held a Black Blade, but someone like Greenbull who seems like a part time swordsman at best, has one.
However, after finding out Fujitora has a supreme blade from Wano when Oden never had one, and Kuzan is a fistfighter who can throw raw punches on par with fucking Garp the Fist. Anything goes for the Admirals. It's no longer unbelievable, might even call it likely.