There is no repentance from blasphemy. Bishop John Joseph commited suicide in protest Pakistans blasphemy laws. The punishment is death but if you don't live in an Islamic country you don't have to worry about that.
"Defaming the Prophet (peace and blessings of Allaah be upon him) is a kind of kufr. If that is done by a Muslim then it is apostasy on his part, and the authorities have to defend the cause of Allaah and His Messenger (peace and blessings of Allaah be upon him) by executing the one who defamed him. If the one who defamed him repents openly and is sincere, that will benefit him before Allaah, although his repentance does not waive the punishment for defaming the Prophet (peace and blessings of Allaah be upon him), which is execution."