I remember Mills and Blooms conducted a survey many years ago: Readers preferred their hero on the book cover to have dark hair, even though the story was about a blond hair hero.
I wonder if that is why we love Colin Firth's portray of Mr. Darcy so much? If you check Jane Austen's original novel, you can see that Jane Austen in fact didn't describe Mr. Darcy's hair colour at all.
Mr. Darcy soon drew the attention of the room by his fine, tall person, handsome features, noble mien, and the report which was in general circulation within five minutes after his entrance, of his having ten thousand a year. - Chapter 3, Pride and Prejudice
It was very clever of Jane Austen to leave the physical appearance of Mr. Darcy to our imagination.
And I'm only joking, I love Colin Firth's Mr. Darcy for his wet T-shirt scene, not just because he's dark hair.
Photo: BBC Pride and Prejudice screen shot