The striker comes to the Green Monster after a season with Norway's Kongsvinger.
He is expected to play in a friendly against Deportivo Savio on Saturday before joining El Salvador as it prepares for this month's UNCAF Copa Centroamericana.
Blanco was a standout with Salvadoran power Isidro Metapan from 2010-2012. During the club's 2011/12 CONCACAF Champions League campaign, the now 23-year-old scored three goals, including both tallies in an historic 2-1 quarterfinal win over Mexico's Pumas.
With El Salvador, Blanco scored three goals in seven World Cup Qualifying games for Brazil 2014.