Most of the FFs are a little too campy to be considered "dark." FFVII certainly has its moments that are "darker" than most (Vincent's subplot, wandering through Cloud's mind in Lifestream). But the brisk pacing, which meant not delving into certain parts of the plot much, results in a game that isn't really dark. I guess it depends on how you define dark, which to me is not just colors and emo-feeling, but requires a certain realism.

Under my definition, FFT is probably the "darkest" game in the series, as it focuses, more in-depth, on the human elements of betrayal.