It is relatively common for series (especially shoujo) for the/a love interest to be blond. This appears in Ouran High School Host Club, Kaichou wa Maid-Sama, Tokyo Mew Mew, and a number of others. Why is this common in Japan, where the population is 98.5% ethnically Japanese--and as such naturally dark-haired--and most of the rest of the population is either Chinese or Korean?
It might have something to do with the representation of the blonde-color. According here,
Blonde hair on a male character may imply that he is a trickster. Alternately, it may symbolize that this character is a focus of chaos, and trouble follows him wherever he goes, such as Vash the Stampede from Trigun, or Soichiro Nagi from Tenjou Tenge; a live-action example is the movie version of Battle Royale's Kazuo Kiriyama. It is also not uncommon for anime males with long blonde hair to be portrayed as Casanova-types. (Examples: Kurz from Full Metal Panic, Allen Schezar from Vision of Escaflowne, Kyle from Suikoden V among others...)
the easy artistic reason is because it makes it that much easier to tell characters apart very quickly. If it's a manga, you won't have voice to tell the difference, and the art style can be very similar between two male characters who are supposed to be hot. If you can't hear the difference, and you can't see much difference, an extremely different hair color makes them distinct even if they're facing away from viewer or are barely in frame!
If it's natural, it's sometimes used to make them seem foreign (usually french!) in a way meant to be mysterious and attractive. If it's dyed, it shows they have an interest in their looks, and often that they're a rulebreaker (since most Japanese schools have rules against changing your hair color), which means it's usually a casanova type.
Well, in Ouran, i know one of the blondes is like, part french, and the other is just suppose to be the cute little one and the blond hair just adds to it, plus in that anime is a school for rich people and we see a lot of them with dyed hair, some background characters have light brown hair or red hair, so it's to say they're rich, and most of them are easily able to dye their hair. Also some of them are part caucasian.
In Kaichou, the love interest is also from a wealthy family and his mother is from England. The japanese people still have very dark hair, and dark eyes.
In Tokyo Mew Mew, he is also a rich boy from Europe. The actual love interest is only blonde when he is possessed by an alien godlike entity.