Sorry, I'm late.
I like the theory, and bottom line I think it's possible that Aramaky didn't forge it, even though by Occam's razor I think he did. In the end, why introduce a kokuto and give Aramaki one when it could have been (even in this scenario) a "normal" Saijo, even though working like Enma?
Ok, Gods now are being introduced and so super strong characters are being introduced, but this forces you to "create" another black blade forger just for Aramaki to have a BB... which for him wouldn't change much for him. It would be totally different if he indeed forged him black.
Also, imho, GB's sword is one of the 21 too, just because I think the Yoru will remain the only black Saijo, but this is merely a guess.