She went back to dark because she likes her dark hair better. Her plan was to always go back to dark as soon as her ANTM time was up. As soon as McKey was announced the winner, her hair started getting darker and darker. I think the dark hair against her pale skin and green eyes works better.
I like her with her neck length brown hair the best, because I think thats when she looks the most high-fashion. Although I suppose if she was to do an old-fashioned shoot, like idk embodying stars of the 50's through 80's her blonde hair would be the best.