The idea of black hair as a female symbol of evil and power is illustrated perfectly by Rachel Wise:
If the witch and the fatal woman have functioned over the centuries as warnings to women who might be tempted to act autonomously or enjoy their own sexuality, the beautiful brides of hero tales have compounded women’s psychological oppression by providing a model of what they ‘ought’ to look like–in appearance, attitude and behaviour. This model has profoundly influenced women’s perceptions of themselves and has contributed to their pervasive self-lingering belief that it is natural for women to be submissive and self-denying, sacrificing their interests to the needs of the men in their lives.
It is hardly necessary to describe the physical attributes of the hero’s bride as her late twentieth-century incarnations smile at us every day from advertisements, fashion magazines, film and television screens, but a consideration of the significance of her appearance is instructive. She is, of course, beautiful; in one of the most popular fairy tales ‘Beauty’ is her only name. But her beauty is of a particular kind, and advances in the technology of printing and the reproduction of works of art, together with the advent of film and television, have made the visual definitions of Western female beauty as familiar as the motifs of the hero story itself. …
To begin with the bride is white, and usually blonde. In fairy tales golden-haired beauties abound; the only memorable dark-haired heroine is Snow White whose hair is in stark contrast to the pallor of her skin. Rapunzel is more typical: she had ‘long and beautiful hair, as fine spun as gold’, which she let down from her tower window to allow the witch and the prince to climb up. … The story of ‘Goldilocks and the Three Bears’ did not become popular until the late nineteenth century when it was modified to emphasize the golden hair.
– Marjery Hourihan, Deconstructing The Hero
Hourihan also explains that even when the blondness of hair is not mentioned in picturebooks and illustrated stories, the illustrator often gives the ‘beauty’ blonde hair anyway, such is the power of the trope. Cinderella has had golden hair since 1854, when the illustrator George Cruickshank gave her them.
See also The Betty and Veronica Trope explained with numerous examples.
The blonde-hair is better than brown-hair idea continues to be perpetuated, as recently as Laura Ingalls and Ramona Quimby, both of whom despise their own brown hair, wishing it could be blonde. However, I take it as a good sign that I can’t think of any more recent examples than Ramona’s. (Tina Fey addresses it in her autobiographical book Bossypants, and explains that as a child she noticed that blonde girls get all the attention.)
Related: The Twisted History Of Snow White from Reading Today Online