That to forge a black blade you don't solely need what Gabban just used against Sommers which is what folks in wano used to carve poneglyphs. Mihawk and Ryuma are one in the same. They are known for 2 things 1) forging Black blades 2) fighting endless battles until there is no one left to fight Mihawk and Ryuma essentially used this technique in countless extreme pressure battles and "forged" black blades