I'm not gonna lie, I've actually thought about this quite a lot. On the one hand, FF leading men tend to have very light hair. Cloud, Zidane, Cecil, Edgar and Sabin etc, while the leading women tend to have dark hair (this is especially true in the later games, no so much the, erm, First Generation ). So, I'm wondering if this is a kind of tongue in cheek stab at their own motifs through the years. They "need" a light haired leading man, so they "make" Tidus dye is hair blonde.

Of course, Slade could be correct here;
Quote Originally Posted by Slade
It is possible that his hair naturally bleached. When you spend alot of time in the sun your hair bleaches. I know from experience (although, it wasn't as extreme as Tidus, just the tips of my dark hair went blonde). Tidus would spend alot of time outside training and such for his Blitz tournaments and thus, his hair would bleach over time. The only thing that suggests otherwise here is that Zanarkand seems to be always dark.........
, which is also something I've thought about.

Is it weird to think about this stuff? xD

EDIT: Additionally, in response to Anaisa, if you start out with light brown hair, it can bleach completely to a dark blonde similar to Tidus's. But I'm still in favor that the developers were poking fun at themselves. :]