I've seen that picture before, and Tidus was never intended to have Black hair. it just turns out that way because his blond hair, when turned into a B&W image, is darkened.
He would look horrible with Black Hair only because we're all so accustomed to his blondish look from already having played the game.
If we would have played the game and he had black hair from the start, I bet we all would say Blond hair on his would look weird.
oh i definitely agree! this is right along the lines of what i was going to say.