Sattar Behechti was imprisoned Oct. 28 for criticizing the Iranian government on Facebook, Radio France Internationale reported.
Behechti said he had been harassed a number of times by security service members over the telephone in the days before his arrest.
"Yesterday they threatened to tell my mother that she would soon be wearing black if I did not shut up," he wrote online.
Behechti's death hasn't been independently verified but his relatives reportedly received phone calls Wednesday in which they were told to pick up Behechti's body from the prison.
France called on Iran on Thursday to explain Behechti's death.
"We are profoundly shocked to learn about the death of blogger Sattar Behechti while in detention. We are calling on the Iranian authorities to explain the circumstances of his death," said Vincent Floreani, deputy spokesman at the Quai d'Orsay.
"France expresses its highest concern for the continuing degradation of human rights in Iran," added Floreani.