No question that Blair was a mean girl, a bully, a busybody, and more. But the girl had some damage. Her best friend, who her boyfriend was uncomfortably obsessed with, left her the same year her parents split up and her dad moved out of the country with a man. The pressure to be perfect led to an eating disorder, and that's just where she's at when the series begins.
Blair's moral compass definitely whips back and forth throughout the series, but her setbacks actually cost her something enough to keep her from ever going too far off the deep end.