Originally Posted by aisle_s
When Riku told Sora that he was just the delivery boy, it seemed to me to be just more mind games to weaken Sora's heart. When he had a moment of weakness the Keyblade seemed to have chosen a new master (at the time, Riku), and then Sora regained it inside HB when he was going to fight with the wooden sword.
I don't think I believe the delivery boy theory.
As for King Mickey's Keyblade, I can't really believe the dark realm theory just because there is no evidence to support it other than the introverted colours.
If the Keyblade that Riku used in KH1 was made of six of the seven princesses' hearts, perhaps that's what all Keyblades are? Someone with a strong heart gains a Keyblade, it represents the strength they hold within. However this does not explain the reason Kairi can wield a Keyblade Riku gives her, but it could.
If a Keyblade is a token of a strong heart, and all hearts contain light and dark, that is what a Keyblade is made of. Perhaps Kairi was unable to form one because she was one of the Seven Princesses, there was no darkness to form a Keyblade?
That is pure invention on my part, but it seems plausible to me.
Hopefully these questions will be explained in a KH sequel (if there is one).