Though Mihawk is aware of himself when he warns that his blade knows no restraint, it doesn’t really give the impression the blade is possessing him. The eye thing could be anything at this point, maybe his lineage is lost and special as well.
When it comes to Blackbeard the other soul probably takes over when he’s sleep. Which is why his eyes are normal in general. Probably the same case with the gorosei where they might have a part of imu’s soul in them but only when he takes over the body they get the ringed eyes
We’ve seen Kaido get ringed eyes when he got more berserk and beast like
Same with cavendish when the hakuba spirit was fighting over control with him
Luffy’s eyes changed too in g5 but instead of them being ringed it’s just a thicker outline. Which imo might tie into perfect unison or something but idk


