Eh. Oda had Zoro using Shusui for a while, so Aramaki having someone else's black blade isn't that crazy. As for who forged it, it could be someone we don't know about yet, or a high class CD (similar to Garling or Nusjuro), or Zoro's father.
But yeah Aramaki probably did forge it, it wouldn't be a surprise since he is a top tier after all, and he seems to be focused on Armament Haki (which is a contrast to Issho having top 3 Observation Haki out of the people alive).
The fact Is that Zoro got Shusui because he was Ryuma's descendent and was meant to surpass him; doubt Aramaki can have the same or a simile plot.
But more so, It's the quote: If he has one, it's because he forged it. If he's not a good swordsman, it doesn't even make sense for Oda give him a sword like that, let alone create a random samurai just for Greenbull to have stolen a sword that he doesn't even use properly.
I mean, if that wasn't his doing, why didn't just given him a normal sword? Even a Saijo I mean, if you want. I wouldn't see the reason.
And you would have still to create another forger only to justify Aramaki having It.
I agree and the focus btw
We'll see, sooner or later.