I tend to disagree with the idea that Imu has Sapphire Scale. If only because Imu would likely be dead from it by now. Maybe people in the past had a higher tolerance for that kind of disease when it was more common. Or maybe Imu's just constantly using a form of the Gorosei's regeneration abilities. But, I still doubt it.
Personally, I'm betting Sapphire Scale is something a little different than what many have been led to expect. I'm linking it more to Dragons.
Look, Kaido is a Blue Dragon. This disease is called "Sapphire Scale". Dragons, famously, have scales. Momo's Devil Fruit is considered a failure because he's pink instead of blue. Am I crazy for connecting these things?
Could "Sapphire Scale" actually be a attempt to clone Kaido's Devil Fruit like they have with other Warlords for the Seraphim? Could this be some sort of attempt to create a army? It's only a "failure" because this augmentation isn't supposed to kill you.
Again, this kind of connects back to my whole overly long "Dragons" megatheory. But, it kind of makes sense. I think, anyway.