Eh true blonds are very rare from what I have seen- tons of people just color their hairs blond- I think that might be the reason why you don't see as many blond guys- guys tend to color their hairs less. Or then for some reason you just happen to have encountered lot of natural blond girls.
I guess it's really usual that hair turns darker with age, my brother was blond as kid as well, but nowdays has light brown hair. I don't really know why though.
That depends where you go. I came to visit Netherland, and there are many many blondes, they seemed to be more than the majority on the streets. I've never seen so many. There were so many blonde folks - especially blonde girls, that I was totally shocked, felt umcomfortabe walking in the streets of this country the first day. It was.. quite homogeneous. I wasn't used to that. One black guy among many blonde folks is quickly noticed lol. To the point I had an irrational distrust at the beginning. ( even thinking about the aryan race and how Hitler would have been happy in this country for few seconds ) But people are open minded and cool over there, so I quickly gave up my sillyness lol.
Even pretty blonde girls, curious, came to talk to me very nicely, the french foreigner, without me doing the first step.
In France I see diversity very much,, the only thing that is a rare sight is redheaded. Which is why I'm always intrigued when I see one on TV. Quite exotic to me. I find Triss much more attractive than Yennefer for example. Her flamming hair certainly plays a role. 
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Otherwise I see that people want absolutely a blonde as Li. Why not. I can only support what we never had before. I am fine with a blonde, redheaded, black girl, asian, or anything else, as long as it isn't brunette. I love dark hair as well, but it's just that I enjoy diversity. We had Ashley, Jack, Miranda. it's about time to show some love to other types of women. =)