They should be taught the concept of "self fulfilling prophesies" . Black hair bring calamity? persecute them and one day they will bring just that out of spite.
Maa its a trope for a reason, besides can we really blame them tho? Especially if the setting is a world where black isn't common besides
Black death, black cat = bad luck, Black bellied, etc. alot of things that are considered "bad" is often related to black for some reason
Probably based on their collective experience. There might be in the past when they experience tragedies and saw a commonality like predators always hunt at night when it is dark so they associate dark or black with death.
In this case , most likely some black hair tribes of alleged invaders invade them. May or may not be true( they are bad) since the victors tend to write histories where they are righteous.
Also possible some black haired clan are made a scapegoat. They might be powerful but some people either got envious or oppressed by them(power tend to do that) and they conspired for their downfall.
In ancient Egypt it is the opposite , they view black as life and
white as death. It is due to white sands of the deserts are infertile and will kill thier crops while the black soil of the Nile gives life.
Edit: Apparently , I made a mistake. White means something else , black is right though, it means fertility.
The swing from "everyone is welcome here" to the Eugenics receptionist.
I worry for she would groom her to be elitist.
Hmm i think the whole Black Hair came from some "Hero" who was summoned/isekaied from Japan and multiple kingdoms fought over him before crossing the line and he blew them up or something (hence the black hair bring misfortune thing).
Also Jesus Christ Japan and their freaking persecution complex over black hair holy shit (Black hair is literally the most common hair color on Earth) can't they choose something else ?.
Possibly like what happen in the Japanese TV series "Dogs of Orthrus" specifically the "Devil with God Hand" . The plot is basically "An Angel with Death Hand and a Devil with God Hand(can heal any maladies)" The reason for the 2 men to have such attributes is their power.
The one with death hand first experienced the fragility of life so he viewed it as precious so he lived a relatively saintly life. While the one with the god hand became cynical and antisocial cause he saw the length humanity will go through just to be healed and will resent him just by not being able to save their love ones.
In this case after they was gone people resent them just by not being there. Just like the scenario of the live action. They able to heal any maladies but when they are gone they resent them for every tragedies. There are other scenarios where they will resent them.