The black hair was for sure a wig. She hasn't had dark hair for about three years, changed it after the Eight Legged Freaks Premiere in Summer 2002.
She just was a bit darker again for In Good Company and Love song for Bobby Long.
IMO she looks best with her natural hair colour, dark blonde - brown:
I hope all of these pictures are from before Summer 2002, please correct me if I'm wrong.