"Blond guys aren't dumb. They're evil. Like in The Karate Kid and World War II." —Bart Simpson, The Simpsons
Fair-haired guys in media frequently end up evil. This is especially noticeable if he is compared to a nice, dark-haired hero or if he is on the opposite end of a Love Triangle. And if his blond hair is long, you'd better hope he's an elf, because if not, that's pretty much evil incarnate. In many ways, this trope can be seen as the less extreme version of the Evil Albino.
If they're not evil, they're still jerks, although they may or may not be nice underneath, and they usually lose. Where a Betty and Veronica usually has a blond Betty and a dark- or red-haired Veronica, a male version of this setup will do the reverse.
Given that blond heroes were not rare in films 40 or more years ago, this may be a Cyclic Trope, albeit with a longer cycle time than that for female "bad hair colors". That said, it has certainly become a much more prevalent trope in the days since World War II, which may be responsible for this, since the Nazis did love blonds. On the other hand, it may be more simple: popular kids are jerks, and so are all cheerleaders. Blonde is a suitably desirable hair colour for both, therefore...
Often synonymous with Light Is Not Good; after all, blond guys are more often associated with an angelic appearance than black haired ones.
Compare Evil Redheads, White-Haired Pretty Boy, White-Haired Pretty Girl and Evil Albino. Contrast Hair of Gold. The Distaff Counterpart is Blondes are Evil.
Poster Comment:
Steve Sailer has written about this.
Not only are blond guys evil in the media, but so are blonde women.
I don't know why but I suspect it's envy from short ugly Jewish guys so prevalent in making movies.
My hair was always blond and it was curious how short ugly brunet guys were so hate-filled toward me.