Roger don't have Black blade as well as Rayleigh, plus talking about supreme grade sword. it was stayted if Zoro paint his Enma in black this sword will became supreme grade sword. But Gorosei have Supreme grade sword kitetsu, even if it don't have black paint.
hes the strongest swordsman who competes for that title. if mihawk never fought sword gorosei how would anyone know who is stronger?
sword gorosei is obviously a powerful swordsman on the top level because of the implication he has Shodai Kitetsu, the most powerful cursed sword in the verse, a supreme grade blade, and has had it for decades without dying, meaning he's mastered it completely.