Sanji at the very least is a prince and of royal bloodline, so he has potential for CoC.
Sanji does not have potential for based on his bloodline. You’re obfuscating royalty with people who take it. There is
one(?) confirmed user of CoC that is descended from
legitimate royalty, and that would be Doflamingo, who is actually a Celestial Dragon.
You can go down the list of characters who have CoC and the vast, overwhelming amount of characters are not of royal blood, and of the ones that are “royalty,” it’s not a legitimate claim beyond calling themselves royal because they
took the throne as opposed to actually being a part of the bloodline.
Sanji's case at the very least is very like Katakuri's. When Luffy (or Zoro) is/are away, he seems to be the primary caretaker of the Straw Hats just like Katakuri. They are CoO specialists of royal bloodline.
Sanji’s case is nothing like Katakuri’s. Katakuri does not have it because he has a royal bloodline. He does not. He has it because Big Mom has it, who is a titan among people in the series, and his own drive.
Also, the point about “caretaker” is questionable. The Straw Hat’s have split up one time into different groups. It’s not some predominant theme with him. They all do their own thing.
It has nothing to do with actual royal bloodlines, based on everything we’ve been shown.