Doesn't make any sense at all. 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.
You could not defend the claim it doesn't make sense if you tried.
Zoro received Shusui and used it for damn near 500 chapters. He was a good swordsman before he received it.
I didn't say about anything a random samurai. Who's to say it was a samurai at all? They're not the only ones with black blades. Who's to say there isn't a specific lore about that sword yet to be addressed?
Your assumptions don't make sense, because they're not rooted in reason. They're rooted in dislike for the notion Aramaki didn't forge the black blade himself.
Just say that you don't like that idea instead of pretending you put any thought into it.