Curious. What's "That level" for you?
Like we legit don't have an answer to how a Black Blade is forged.
Enma is the biggest indicator on how the process most likely works, as it physically absorbs a person's CoA. That's most likely the key to forging a black blade.
I've said it time and time again. I believe this is the same process that was used to create the Poneglyphs. Black blades are considered indestructible, just like the Poneglyphs. The samurai are known for their CoA, and the ancient Kozukis were probably leagues above most with CoA. Zoro, who will forge a black blade, is a CoA specialist. Ryuma, who forged a black blade was a Samurai, who again, are known for their CoA.
In my opinion, forging a black blade isn't a matter of overall strength. It's a matter of one's overall CoA level, and an Admiral can easily reach the pinnacle of CoA.