Another plague was on its way, and this time Paraoh had no advance warning.
HaShem told Moshe and Aharon to take fistfuls of ashes from the coal used in furnaces. Once their hands were filled with the black soot, Moshe took all of it in one hand and threw it, with great force, up toward the sky.
HaShem then spread the ashes over all of Egypt. When it landed on people and animals (the ones who survived the fifth plague because they were in stables), it caused their skin to develop a rash of large blisters... OUCH!
This time, it was HaShem who hardened Paraoh's heart so that he would not listen to Moshe and Aharon, and he did not let the Jews go.