The historic trade agreement signed Friday in Brussels is a massive shot in the arm for the four-nation South American bloc known as Mercosur. The trade alliance grouping Argentina, Brazil, Paraguay, and Uruguay has struggled for momentum since it was set up in 1991. Though the agreement eliminates tariffs against the EU in key sectors, the bloc still has to get its own house in order, according to analyst Nicolas Albertoni of the University of Southern California. “Nearly 30 years after its creation…