Blond hair makes her look like a sell-out (i know it sounds mean), like she's mean and common. Maybe they were going for that look for The Roommate.
Brown hair softens her features and compliments her skin tone and eye colour. I always thought she looked like a mix between Audrey Marnay (her nose) and Britney Spears (her eyes).
I hated "Blair's" hair during the whole season 2, those stupid highlights and the redish brown underneath. They made her lose her edge, her elegance. It's like they wanted to make her look like Serena.
/end rant