Hi there. I was recently reviewing chapter 955 and something got me thinking.
At the end of the chapter the swordsmith Hitetsu Tenguyama said to Zoro that Emna is not yet a black blade. We don't know how black blades are being forged yet.
My theory is that they are not forged but elevated through haki to that status. If a user imbues strong haki into a blade for long periods of time then haki will merge with the sword strengthening it and turn it into a black sword.
Now we don't know why Oden didn't achieve this with his swords in the past, but I believe because he had monstrous power from the beginning he wasn't forced into a corner to many times. He didn't have the need to strengthen his swords to the limit.
Thank you