I agree that, most likely, nothing happened to your brew. You may have picked up a little oxidation, but not much is going to diffuse through the little hole where the tube was.
However, I must ask why did you still have a blowoff tube connected after 13 days? Unless it's all dirty, it's no big deal one way or another, but I've just never heard of it.
By the way, that's one heck of a beer. I hope it works out great!