"Her sleek black hair framed her cheeks,"
Nicole Kidman
Creative Commons Attribution Share AlikeNicole Kidman - Credit: Georges Biard

In the novel Mrs Coulter is described as having dark hair, but in the film adaptation, The Golden Compass, she was played as a blonde by Nicole Kidman, who was always Philip Pullman's first choice for the part.

Given that physical details are rarely forthcoming in the novel (it is a long time before we learn that Lyra is blonde) this is an attention to appearance that is quite fitting for the character of Mrs Coulter.