Quite the innocent question as it doesn't seem to serve any purpose except roleplay, but I'd like to address these questions for the sake of ... science. And also because a player brought the project up :
1 - Do Intelligent items possess a soul, or rather some sort of artificial intelligence?
2 - Does a Magus' blackblade, while clearly defined as a sentient magical weapon, count as a familiar for the purpose of the Soulswitch spell?
3 - If it does, how is it represented in game terms? Does the character's mental stats change to represent the Blackblade's, for example? Would the blackblade be bonded enough to be able to use the Magus' abilities?
4 - And should such a shenanigan happen, should a DM take over the "Possessed Magus" until the true self comes back as a standard action? After all, the blackblade should be considered a NPC.
Intelligent Items Flavor Text
Magic items sometimes have intelligence of their own. -->Magically imbued with sentience, these items think and feel the same way characters do and should be treated as NPCs.<--
Targets : -->you and your familiar<--
You may only cast this spell if you currently have a familiar. You place your soul into the body of your familiar, and your familiar's soul is placed in your body. This functions as if you possessed your familiar using magic jar; your familiar simultaneously possesses your body in the same manner. You can't freely transfer your soul between your body and your familiar's—if you choose to return to your body as a standard action, the spell ends. If either body is killed, both spirits return to their original bodies, the spell ends, and the original owner of the slain body dies.
At 3rd level, the bladebound magus’ gains a powerful sentient weapon called a black blade, whose weapon type is chosen by the magus (see sidebar). A magus with this class feature cannot take the familiar magus arcana, and cannot have a familiar of any kind, even from another class.
A black blade is bonded to a particular magus, -->much like a familiar<--, but in more of a partnership than a master-servant relationship.