What would be the point of kidnapping a blond girl, then making her look like one of them?
I thought the suggestion was that blond girls are kidnapped in order to earn money begging, as blond children are so unusual in that region, so bring in more money. Surely hiding her blond hair would make the kidnapping of her pointless.
If her hair was dyed to disguise her, then I don't think she was kidnapped specifically for being fair haired.... if she was kidnapped at all.
Maybe her hair was dyed because she really was one of the community and they treated her the same? if she was just a poor child from a neighbor or a poor gypsy child who happened to be blonde who they took in, they would just henna her hair like they all do.