I don't remember much, but the basic premise of the fic is that Kurt has been receiving threatening letters (?) and someone (either Rachel or his manager, I think) makes him hire a bodyguard. Blaine was working as a part-time bodyguard for Cooper and that's how Kurt's people find him. I remember lots of delicious pining and angst and Blaine keeping himself distant (calling Kurt Mr. Hummel) because he knows that if he lets him get close to Kurt he'll fall for him and their relationship needs to stay professional.
Any help finding this lovely fic would be much appreciated (:
EDIT: Ahhh, I'm such a loser, I found it pretty much right after I posted this.
To Shield and Protect by afterthenovels on Scarvesandcoffee.net