I think the final nail in the coffin for TnB was when Todd saw Blaire and Spencer going horizontal at the cabin. After that, Blaire become little more than a tool - someone he uses when he needs, someone he once loved, the mother of his kids, but certainly not someone he's in love with. I think he loves Blaire but is tired of her games and her jealousy and pettiness so other than a fondness because of their past and the connection they have through the children, he just doesn't have time for her anymore.
I love it too! It's interesting how all of Blaire's non-Todd pairings have been such huge failures and yet Todd has had success with Blaire, with Vange and with Téa and all three pairings were really successful and really popular. People still talk about Todd and Vange. Blaire screwed up way to many times - bedding Cristian because she thought Todd and Vange were together, Spencer, John and who knows who else. I can't even keep track of all the guys she's been with.