Responses
It's probably just an uncommon color compared to the unique colors of the rest of the people. It's probably common among regular citizens and maids but most royalty didn't have black hair back then. There's also the thing where black represents evil and stuff so they're just reflecting that onto her.
The same way some people considered red hair as witches and the same way some people considered smart/wealthy/'rebellious' women as witches irl. Either a history of someone 'wicked' with black hair in past or it is an extremely rare colour there.
I have black hair but that doesn't mean i know how to curse someone. Where did they got the idea that people with black hair are witches?
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