I thought about those things too but I still think Blair has enough mobility to guard and close out on most perimeter-oriented bigs in the NBA. Dirk is obviously the most difficult and extreme of those match ups yet we've seen Chuck Hayes give him hell at times. Blair is better than Chuck in pretty much every facet of the game including mobility, lateral quickness and all the big categories for judging a player's defensive potential in pro's. It could work, IMO.
I'll watch him a few more games. Forget the extreme example of Dirk, Blair would have a difficult time staying in front of any PF with a floor game of any sort. As a defender, I don't think he's quicker than Chuck. As a post defender, Blair could excel. But if he makes it into the NBA as anything other than a backup C, I will be shocked.