After serving in Warsaw with the SS, he joined the "Sicherheitsdienst" (SD; Security Service), a division of the SS. In 1941, he was briefly transferred to the Eastern Front, where he served with the "Einsatzgruppen" mobile death squads. He participated in executions in the occupied parts of the Soviet Union, before being transferred back to Warsaw. He served in the SD's Warsaw ghetto outpost in mid-1942, when the mass deportation of Jews from there to the Treblinka extermination camp began. Blösche hunted down many Jews who were hiding from deportation. In January 1943, during another wave of deportations to the death camps, he took part in another search, which also involved frequent murders or executions. He participated in the shooting of about 1,000 Jews in 1943.