Blonde hair turns brown as kids get older?

I like babies and toddlers very much. I sometimes see blogs that parents put on their kid's photos. They are ve--ry cute!

There is a thing I wonder. Although both father and mother have dark brown hair, their kid's hair is blonde. Why? It is not uncommon? I don't know about it, because I was born and grew up in Japan so most people around me have black hair. (There are many people who color their hairs though.)

I wonder if blond hair is recessive heredity like blood type O. Or it turns brown sometime?