Even if both your parents have dark brown hair, but if they each had one recessive gene for blond hair, then they would pass along to you the genes to have blond hair.  What this actually means is that both of your parents did not have the dominant genes for dark hair, so they could not pass the trait along to you.  If one or both of your parents had gotten their dark hair from dominant genes received from their parents, you would not have had any chance to be blond.  If even just one of your parents had passed along a dominant gene for dark hair, your chances of being blond would have been reduced by 50%.  For certain traits to be passed along to children, the parents have to have dominant genes for those traits.