Blasphemy must be allowed. Though, not always appropriate, it must always be considered acceptable.
Because everything is blasphemy. Every Christian church blasphemes against Islam and Hinduism and every non-Christian religion (and maybe even some different forms of Christianity too!) every single week. Every religion blasphemes against every other religion, and just about every statement of opinion is blasphemy to someone. Hell, even demonstrable facts can be blasphemous (consider evolution).
Something cannot be considered wrong simply because it it blasphemous, because otherwise, everything would be wrong.