I disagree — I think Mihawk could pick up any rusty katana from Marine base G69 and eventually make it a black blade — yes the quality of the swords matter for swordsmanshi (e.g several of Zoro's swords breaking under his strength, Shusui being able to produce stronger air slashes (and swallowing up the slashes of the weaker swords)), I'm just inclined to take Mihawk more literally. Mihawk said that with haki any sword can become a black blade, and I'm inclined to believe that; I don't think you need meitou to forge black blades.
Only two we know of. Black swords were famous enough that even preskip Zoro and Tashigi knew of them. Shusui had been with Moriah (and before that hidden away in Wano), and given the hype around black swords, I'd be surprised if Yoru was the only reference point the had.