Blasphemy is not the same as criticism. On one extreme of criticism, you use reasoned academic arguments to critique specific doctrines you find unappealing. On the other extreme, you can insult people's culture without being productive at all.
Blasphemy is much closer to the second than the first.
Though i think you're right, usually blasphemy is just insulting, is not always the case, defending the theory of evolution is considered a blasphemy by a lot of people. And don't think that is insulting.
Blasphemy is much closer to the second than the first.