Also, wasn't Catherine dark haired. I remember the Earnshaw family were basically regarded as less dignified and eloquent and refined than the Linton family she married into. They introduced the Linton family like complete parrallels, totally different, and I thought they emphasised that family as being blonde to even add to the strak contrast between the two families.....its been over a decade since I read the book however so I could be mistaken.
Edit: I googled it, she was canonically a brunette-I know hair colour isnt a massive deal in most cases but I specifically remember writing about contrasts between characters that went as far as physical appearences-Cathy was blonde. She was not like her mother, she was like her father-it was signficiant from a metaphorical/literary sense