From what I have read (my main sources are the Way of Kings prologue and Mistborn), It appears that physical objects can be stored in the Spiritual Realm. See: shardblades, Ati, Leras, Vin, Sazed. Their bodies were no longer physical when they didn't manifest themselves as such.
Now, this, to me, implies that physical objects can be 'stored' temporarily in any of the realms, and if this holds true, intellectual and spiritual objects can be stored in any realms as well. Further evidence: Memory can be stored in copper by a feruchemist. I am of the opinion that once memory is stored there, the coppermind becomes an access gate to the intellectual realm where those memories exist. It's either that, or the memories become 'intellectual' objects stored in the physical realm.
Am I making any sense?
Carrying on. There are other metals that you can store physical attributes in: weight, strength, etc. It seems to me those physical properties are stored in another realm, and when accessed, can return to the feruchemist they are bonded to, just the way a shardblade can return to the person it is bonded to.
Now, Nightblood is a physical object that has had a personality bonded to it. For that to happen, my theory is that an awakener would actually spend breath to open a permanent access gate to each of the realms, and with a command, select the personality that the bonded object receives.
Also, pulling physical objects from the spiritual realm seems to be cold. but that's rather unrelated to the rest of my post.
Anywho, please ponder and discuss. Oh, and shoot holes in this so that they may be filled by glittering, tasty blood, please.
Disclaimer: the views and opinions expressed by Valkynphyre do not necessarily reflect the views and opinions of Valkynphyre or any of his associates. Tresspassers will be shot. Survivors will be shot again.