So, It's not shiny or regular, but it has light blue horns, and a mostly black and blue body. What is the reason for this?
The form you're describing is known as Neutral Mode, and appears when Xerneas isn't in a battle or Contest. This form difference is purely visual and has no effect on gameplay. The developers probably did this to make Xerneas look stronger when it reawakened.
Xerneas (Bulbapedia)
Pokémon X and Y had problems with colour saturarion. This sometimes caused some Pokémon to have slight differences in color in these games then their normal models.