Keyblade Alignment

[Teldranis]

Keyblade users are not simply "users". They align their powers and abilities depending on the alignment shared by their heart.
The power of the keyblade tends to allow for them to use all of the natural elements - such as fire, ice, and lightning - much like a master would, but the keyblade is a weapon that follows the users heart. As a result, the abilities they can grant can vary from Darkness to Light as the user himself does.
Keyblade users must, as a result, eventually choose between a dark or light alignment for their Keyblade. This choice is fairly permanent - if one's heart were to waver and shift to the other alignment, the Keyblade itself would second-guess it's choice of wielder and simply refuse to be wielded by the user any longer, making the user a weaponless regular in power until the user corrected himself.

It should be noted that "Dark" keyblade wielders are not by necessity evil. Darkness is considered a necessary part of the kingdom hearts world, and one who chooses to manipulate it while maintaining caution while tapping into it, rather than being intoxicated by it, will be capable of using darkness without being warped to evil by it.

Only dark-aligned keyblade users are capable of using darkness-based techniques, and only Light-aligned Keyblade users can use light-based techniques, which is the extent of the effect that this has on the user's capabilities.