Mickey's Keyblade is assumed to be from the Dark World. Every World (series of planets, whatever you wanna call it as a classification), has a primary Keyblade of its own.
I don't remember the game say anything about each world having a Keyblade.

Riku was the original Master, but if you notice, when the Keyblade goes to Riku, it becomes the Kingdom Key again, because Sora has all the Keychains.
But I /do/ remember the game saying something about that along the way...but in the first one. Could you tell me whereabouts this happens? And if Riku is the true Keyblade Master why can Sora wield one?