Blue kryptonite makes its first appearance in Superman #140 (August 1960), when Superman and Supergirl take on the entire Bizarro world of imperfect Supermen and Lois Lanes. This time a seemingly human child is born to Bizarro No. 1 and sent to Earth, where Supergirl helps care for him.
Supergirl falls into the path of the duplicator ray and creates Bizarro Supergirl. Eventually an army of Bizarros show up on Earth’s doorstep to conquer Earth and take back the child, so Superman creates blue kryptonite, which is lethal to Bizarros but harmless to anyone else. It accidentally kills Bizarro Supergirl – Superman apparently feels no remorse about this. Maybe Supergirl’s death in Crisis on Infinite Earths was karma?
