I specifically want to know which manga or anime started the trend, so I can reason from there to the how question, in order to not make general assumptions.
There was a time in manga/anime where blue hair was abnormal. For example, from the short clips I've seen of the anime "アタックNo.1", it contains mostly black-haired girls, a couple of brunettes and a sparse blonde. No one there is pink/red/blue-haired. Same with Princess Knight.