As someone who read the translation scan of the light novel, she says that after she receives the blessing from the light spirit, her skin turns halfway from white to moreno (I think that’s how the skin color is named for when it’s too white to be black but also too black to be white), so according to the light novel her skin becomes a more whitened moreno skin. And her treatment after that is due to the BLESSING OF THE LIGHT SPIRIT, not the skin color, the nobles want the prestige of the light spirit that’s why they change how they treat her