Final answer:
The term used for Poles who blackmailed Jews during the Holocaust is 'Szmalcowniks'. They threatened Jews in hiding with exposure to the Nazis for money.
Step-by-step explanation:
In the context of World War II and the Holocaust, the term used to describe Poles who blackmailed Jews during this period is Szmalcowniks. These individuals exploited the desperate situation of Jews hiding from Nazi persecution by demanding money or other forms of value under threat of turning them over to the German authorities. This reprehensible practice added yet another layer of danger and oppression to the lives of Jews in Poland, who were already facing systematic persecution and genocide at the hands of the Nazis.