At the start we have to go back into prehistoric history of at least 11,000 years ago. The theory is that we all had one’s black hair and with the emigration from Africa to Europe these changed. Northern Europe has fewer hours of sunlight and at that time was covered in ice. People who lived there underwent a genetic mutation that resulted in depigmentation of skin and hair. Because it allows greater penetration of the skin by ultraviolet B (UVB) sunlight. (Which is needed to synthesise provitamin D3)!
That means because of the process of losing a whole range of pigments, more and more people were born with different hair colours and even became blond. Blond is the last pigment and people with blond hair look healthier, often better, younger and in general are different looking and outstanding from the black group.
At the same time in the Ice-age of Europe life was hard and dangerous and men died in large numbers when hunting for food. Hunting Bisons and Mammon’s. So, the theory goes that there were too many women competing against each other to win over male partners, as too many men had died hunting.
And in men in later centuries, blond men were seen as strong and protective, as they showed that they could fight off dangerous marauding bands of people who were attacking villages at that time.
So, sexual selection for survival played a huge role in making the hair colour blond as a dominant factor. Blond men seem to be good fighters and protective of their families. And blond women are seen as younger, healthier, and outstanding from the group of dark-haired women. They stay longer young it seems. Many do not get the amount of grey hair as dark hair people. Even dark blondes have less grey hair then dark brown hair. The skin ages slower in many.