If something has a dark tone, it's because of the author's attitude toward the subject matter of the story. Looking at blood or body horror and saying it's dark is making a very superficial judgment. Army of Darkness, Monty Python's Holy Grail, and several zombie and horror movie spoofs are gory and full of blood but not particularly dark.

There's not really enough in the FFXVI trailer to speak to the author's attitude. However, I do agree that there is a dark feel to it.

The FFXVI trailer comes of as dark because of the subject matter. Through a few bits of dialogue, the writer creates an atmosphere of fear, as the characters anticipate a building conflict, and everything that comes with it. There is talk of assassination, a budding conflict that is very obviously a war, a battle over future resources that may mirror crises we have faced in our own world, and may face again in the future.

Through imagery, scenes of battle, talk of war, and a moment in which a fallen dragoon is rolled over onto their back, the trailer creates a sense of ominous foreboding. For the empire to have such a fearsome agent, it must be a truly powerful and dangerous beast, once again building an overall sense of doom.