The question for “why can’t Ryokugyu have a black blade” that I have is really more “why would he have one and not Fujitora?”
Of the two, Fujitora has clearly been portrayed as the swordsman. Every time he fights he uses his (as we now know Supreme Grade) sword.
Hiw would Ryokugyu have made his when Fujitora failed, despite the fact that Fujitora uses his sword in every battle and Ryokugyu doesn’t?
You can come up with tragic stories of Ryokgyu being a swordsman who hides his true power due to the suicide of his lover etc etc, but really, when it comes purely down to the two‘s fighting style as shown by Oda so far, Ryokugyu being the better swordsman than Fujitora makes zero sense.
Then I'll try to answer seriously, even if there Is nothing really new to say for my part.
1) First of all, BB or not BB, Ryo Is
clearly a swordsman: the dude has the same name of a swordsman, looks like him, act like him and has
a bloody sword dangling from his hip. I mean, connect the dots lol, BB or no BB.
If anything, Kuzan being a brawler and having a strenght comparable to prime Garp was astronomically more unlikely.
Now that we have addressed that fact that he Is, we can go and see that in the original film, Ronin Gai, where Issho/Fujitora/Zatoichi, Kizaru and Gennai Aramaki were, Aramaki was an incredibile swordsman that no one had seen using the sword:
1-Set in the end of 19th century, where the Meiji Restoration just started, the hero, Aramaki Gennai (Karasawa Toshiaki) does not act so much like a "hero". He's a drunkard ronin (masterless samurai) who loves to lazy around in the house all day, and depend on his girlfriend for money. He doesn't care at all about anything or what happen in the world. Rumours has it that he has an unbelievably excellent skill of swordplay, but no one has ever seen him demonstrating it.
Is that strange thinking that Oda could be going for the same trope, more so because we know that he Is very likely to be a swordsman anyway?
Especially after Aokiji showed to a brawler only at chapter 1088, 700 chapters after his introduction and we hadn't seen him using punches basically ever before that moment? It's that difficult to imagine Oda is keeping It for a big revelation?
2) about Fujitora.
Fujitora Is a swordsman, sure, but being blind means pretty clearly that his focus Is CoO, not CoA. Forging a black Blade probably takes more than just CoA, but still, imho, It's likely to be a peak CoA feat at the very least.
Now, if Fuji could forge a BB too, you would have an Admiral with a Saijo forged black by him, a peak DF, AdvCoC (as every Saijo user has) and even a peak CoA: you would be talking about someone on the level of Roger, WB or Garp. He would have been the WSC/WSM, not Kaido or any other.
And as much as I love Fujitora, that was not possibile.
It's far more likely that Fuji Is focused in CoO and Aramaki in CoA. Two swordsmen with different focuses.
He’s also pretty explicitly an Akainu fanboy who loves his approach to things.
...
You could never imagine Kizaru saying what Ryokugyu there did. Him and Akainu are partners, Ryokugyu comes across as far more subservient.
And this too, imho, Is another example of how things get exaggerated when Ryo Is the topic.
He Just said he likes Akainu way of doing, that's It. Still,
two times we have seen him and two times he was going against his orders.
You have seen Zoro going against Luffy's orders lately? Benn? Ray?
They have all expressed far more "love" for their captain (well, maybe Benn not for now) and are way more loyal. Are they fanboys? Are they far more subservient?
Ryo simply likes Akainu's way, and that's It.