I may be alone in this but I actually get why Blair would go to Dan as a friend instead of Serena. I have a friend I have known since diapers (quite literally since we were born two days apart at the same hospital) and sometimes people who have known you a long time make it difficult for you to change or feel that have earned a certain control over your life. I wish that is the way they had went with the Dair relationship, purely platonic because they have had great moments as friends.