i think light(ness) or light coloring being associated with good, is similar to right handed-ness being associated with good.
Natural blonds tend to darken with time (not all but most) so truly light blonde hair was sort of biological marker for youth.. fair skin, often meant blemish free-- again, one of those old fashioned biological markers for good health.. and the lighter skin, the more a blemish will show. so to be fair (lightly colored, and free of blemishes.. with light colored hair, was a sign of youth and good health.) just the thing you want in bride, since a young healthy woman will bear children easier..
until very recently, it was very hard to bleach hair.. but many vegetable will dye hair a darker tint, walnut oils, henna, and so on. so a woman getting on in age, could more easily hide her grey hair if she had brown or chestnut red hair.. but a blonde couldn't..
and with out clorox -- white, was a very hard color to get.. linen, wool and cotton, were not pure white.. but rather shades of beige. whiteness required effort..
