Rank is based on the quality of the blade, no? The grade would only go up due to the quality being better than it once was. Hence why they jump a rank
Black blades are stated to have crazy durability and Shusui boosted Zoro’s attack power greatly as stated by Zoro himself
Black blades are stated to have crazy durability and Shusui boosted Zoro’s attack power greatly as stated by Zoro himself