It was never explictly stated Mihawk forged Yoru in the manga, no?
What if his daddy did it for him?
If Mihawk didn't do it, then how come he said "Any weapon can be turned into Black Blade like Mine"
This basically means he knew how to do it.
I mean its not vague anymore, i think even Sukiyaki might knew how to forged any weapon, but didn't tell us cuz Oda had an excuse of Didn't had any time to include this in main story lol.
And so far Mihawk weapon are Yoru and the small knife, but we knew which got turn into Black Blade right?