[1] He left the United States to establish a Communist organization for children, the Pioneers, in Mexico, where he adopted the pseudonym Rosario Negrete.
Working with Manuel Rodriguez, Blackwell founded the Mexico's first Trotskyist organization, Oposición Comunista de Izquierda ("Communist Left Opposition", or OCI) in 1933.
Teachers Luciano Galicia and Octavio Fernandez withdrew from the Communist Party to join the group in 1934.
The group sent Blackwell to become a POUM combatant at the outbreak of the Spanish Civil War.
In 1954, he sought to return to activism with fellow anarchists and friends Sam and Esther Dolgoff with the founding of the Libertarian League.