UN Soldiers, Term derived from the characteristic blue helmets.

Term used in online gaming to signify a famous person playing games under a fake name. Term was coined by Warcraft legend "Shlonglor."

A term in video games that denotes an experienced or decently skilled player in any way (aim, strategy, knowledge in the game, etc) that plays as or creates a new account in a game to be lower-level with the intention of playing against lower level, and therefore lower skilled players, (or "noobs," colloquially). This term also gets loosely thrown around in gaming, since there is nothing wrong with this action, and there's no way people can "catch" smurfers. Maybe it could be an actual new player that quickly grasped the game, or a natural at that specific type of game.

meth heads or associates of meth heads that go around different drugstores to buy pseudoephedrine to sell to those cooking crystal meth

A blue-shirted agent of the Transportation Security Administration.

A "Smurf" a person or group of people ranging in age from a baby, toddler, child, teenager to adolescent, or simply under Parental control.
The word "Smurf" is taken from the comic and TV "The Smurfs" who's many different characters mimic the behavior of today's young adults. The reason behind the Word is that all young adults will cause adults to swear in frustration, to use blue language and because of this "Smurf" if the perfect word to describe all young adults.
See Wiki reference for list of characters and further details at en.wikipedia.org/wiki/The_Smurfs