You need to breed for it. Take the man and woman with the lightest brown hair colors, and get her pregnant by him.

You need to climb up the scale to blond; the hair colors are in the order of black, brown, light browns, the females have a couple of purple-hairs, then red, a couple of dark blondes (You might mistake them for light brown) and finally blond.

As a guide, go to the images folder and open up Male_heads and Female_heads. Just keep breeding the man and woman closest to blond (the bottom) on those images, and you'll creep into blond territory. The child will generally be right in between the mother and father on that scale, but occasionally they will be one or two up or down the scale from that average. (And rarely, they will be, apparently, totally random.) That means that if you mate two villagers that are VERY close together (For instance, both the man and woman the 15th head down from the top in those images) then you will occasionally have children that's blonder - or darker - than either of them.