Its a black blade unmatched in its durability and it belonged to legendary figure someone called a Sword God ?
There is a very logical explanation and it has to do with the blade grade system and raising a sword rank.
First, we must understand how the meito blade grade system works. Some believe Oda's idea for his meito blade grade system comes from the Kaihō Kenjaku, a book of famous swords. The swords are graded by the sword quality and cutting performance. The better quality and cutting performance, the higher the grade. If that's true, than the meito blade grade system would work the same. A meito blade grade would be determined by the blade quality and cutting performance.
Now since we understand the meito grade blade system, let's look at the creation of blades and black blades.
Who make blades? Blacksmith create blades and since they create blades that means the blade grade is determined by the blacksmith. The better quality the blade, the higher the grade.
In Wano, we learned black blades are created by swordsman. Forging a blade into a black blade would have zero effect on the quality of the blade since the blade quality is determined by the blacksmith. It does affect a sword strength. IE: Shusui was an excellent grade blade and when it became a black blade it became an excellent grade black blade.
What does the manga mean when it said a sword rank can be increased by becoming a black blade? It means you can take a blade from whatever grade it is to a black blade grade. That is the new grade. Black blades are stronger than normal blades, regardless of the grade. Black blades have much higher durability and are harder to break than normal blades. The manga told us black blades hardness or durability are unparallel.
Now to answer your question, Ryuma's sword wasn't a superior great blade because forging it to a black blade would not have increased the sword grade that way. It grade remained the same, it just became a black blade excellent grade blade. Which is still superior to a normal supreme great blade. With the only black blade at the time, his blade would have been the strongest blade at the time.