In the April 1940 Norway was over-run by German troops. Blackstein fled to Sweden where he was initially interned for several months, before being given permission to live in Uppsala. In his Swedish exile he found time to refresh his college studies, revisiting topics in Economics, History and Literature. He also found work as a journalist. Along with that he was able to stage several plays by Danish authors. Working with an Uppsala University student theatre group he was deeply involved in the translation and production of several plays by Bertolt Brecht. On top of that, in 1943 he was employed by the Racial Biology Institute at Uppsala University as a lecturer and archivist. On the political front he was also an active member of the Swedish based group of German trades unionists (" Landesgruppe Schweden der Auslandsvertretung der deutschen Gewerkschaften (LDG)"). Within the LDG he opposed the mainstream view, manifesting a certain mistrust of the motives of the allied powers. He was also uncompromising in his opposition to any whisper of a united front between communist elements and social democratic elements in the LDG.