Onimaru made it very clear to Zoro how Black Blades are forged (through countless battles). Ryuma and Mihawk share their countless battles in common and both their blades are unscathed and in pristine condition.
Admiral with a long history of battles against top tiers (only way to weld BB) would've raised a lot of attention and yet Greenbull doesn't have the same recognition and aura as Akainu, Kizaru and Fujitora that everyone fears the most.