A bit of wild theorizing here. Warning:
block of text approaching!! 
What
is a Keyblade? We know it to be the central plot device of the KH universe, the enigmatic and powerful weapon that chooses its wielder, can lock or unlock the passages between worlds, and is especially potent against Heartless and Nobodies. At some point in the past a great Keyblade War raged for control of the χ-blade and, through it, Kingdom Hearts. The Keyblade Graveyard, along with Xehanort's own words, testify to the fact that those who fought the Keyblade War came from countless worlds in both the realms of Light and Darkness to vie for dominance.
I am about to make a very bold claim:
we have all the evidence we need to unravel the mysteries of the Keyblade. Consider the following:
- Keyblades respond to hearts, judging them worthy or unworthy to wield them.
- Keyblades can be used to connect or isolate the worlds, which are known to themselves have hearts.
- Only those with a heart can use a Keyblade. Roxas, for example, can only use Sora's Keyblade because it is tied to Sora's heart, as is his own (non)existence.
- The Keyblade attracts the Heartless.
- The Keyblades in the Keyblade Graveyard are described by Xehanort as follows:
If we consider that this is spoken by a Keyblade master and a seeker of forbidden knowledge, it suggests that:
- Keyblades are, in some sense at least, 'alive'.
- Therefore, Keyblades can 'die'; what is left behind is merely the form of the weapon, not what made it powerful.
- Keyblades are not merely instruments of Light; they can be tools of Darkness as well.
Consider for a moment what
else in the Kingdom Hearts series is described in such ways. Light and darkness... connecting to people and worlds' hearts... an attraction for the hunters of darkness who hunger after the hearts they have lost... and, perhaps most significantly, a source of great power that would be available on most, if not
all, worlds.
It's painfully obvious now, isn't it?
The Keyblade, if my theory holds true, isn't just some legendary magical weapon. It's nature is magical, yes, but magic is specifically
not given the characteristics in KH that the Keyblade and a certain other element are demonstrated, time and time again, to possess. It's quite simple really, in the final calculation. If we ignore Keyblades for a moment, what is the
single most powerful thing in Kingdom Hearts?
Hearts, of course.
I therefore posit the following:
Keyblades are weaponized hearts. It's the only reasonable explanation, given the evidence. Consider that, for the Keyblade War to have happened, the knowledge and resources to
create Keyblades would have had to exist on roughly at dozen worlds at the very least (to match Xehanort's description), both in the Realms of Light and Darkness. This eliminates Kingdom Hearts as the source of Keyblades
en masse -- and confirms
why it is connected to the χ-blade. The χ-blade is the weapon of Kingdom Hearts -- summoning the χ-blade triggers Kingdom Hearts to appear because KH
is it's power source. Of course, this means...
... that, at some point before the Keyblade War, the very first Keyblades had to be fashioned. Given their characteristics, only the strongest hearts could have granted a Keyblade its power. Story wise, these were likely the hearts of great people: kings, heroes, champions of Light and Darkness. Magics lost in the war were likely used to bind the heart to a vessel, creating weapons that could resonate with and empower -- and draw power from -- the hearts of their wielders. If we follow the 'Keyblades are hearts theory logically', we must also conclude that
Keyblades are people.
We must conclude that
people were sacrificed, or willingly sacrificed themselves, to create -- to
become -- Keyblades.
Quae non prosunt singula multa iuvant -- 'what alone is not useful helps when accumulated'. These are the conclusions I have reached through accumulating the in-series facts on Keyblades and hearts. I doubt we'll be seeing anyone become a human sacrifice in the series... but -- and I'm calling this one right now -- don't be terribly shocked or surprised if, at some point, poor Aqua resurfaces only to be "gravely wounded" and, in a shimmering cloud of sparkly light, Sora, Riku, or Kairi suddenly gets a new / upgraded Keyblade. Square Enix would do that, and
you know it.
The Keyblade... has been
unlocked.
