I disagree with the idea that humans have "natural rights". We don't. That is to say, insofar as a right exists, it exists because we agree to create it. You can claim a right to life all you want, for example, but neither a lion nor a microbe will care. Rights exist only as constructs, as general rules for ways in which to treat others in order to have a society where we are better off.
As such, the idea that slander and defamation are something that we have a right to be free from is itself a construct to be questioned. My belief, and a belief usually held in the U.S. court system, is that something only functions as slander or defamation if it is not true. So the idea of blasphemy laws as a defense against an assault on the beliefs of others is already fairly questionable, under that rule.
One could argue that the feelings of others are more important than telling the truth, but this is an argument that most people don't seem to personally desire to have inflicted on them in the majority of cases, and, moreover, tends to become pretty impractical pretty rapidly. Personally, I value knowing truth quite a bit more than feeling blissfully ignorant of reality.
And most blasphemy pretty easily falls under the "it's true" umbrella. For example, I can say that the God of the Bible seems to be a pretty murderous asshole with a penchant for racism, sexism, and sadism. Is this blasphemy? Yes. Is it true? Well, according to the Bible, God flooded the earth and decreed death for several hundred offenses, including violating the Sabbath by picking up sticks on it (Exodus/Numbers). He also had one chosen people, and referred to every other ethnic group on the face of the earth as "dogs" (Gospel of Matthew, I believe). He had an immense number of laws directed at controlling and even oppressing women, all the way through the New Testament, from the death penalty for being raped in a city and not being heard to scream, to having to marry your rapist (if you weren't betrothed), to not being allowed to speak in church. Oh, and he will torture you after death if you don't love Him with all your heart and seek forgiveness for even thinking about something wrong (again, Gospel of Matthew).
That's blasphemy, but it's all true. If that is an attack on those who cling to the Bible as a textbook for belief, well then - and I say this as someone who once did also so believe - good. If your beliefs can be truthfully explained with the effect of making you feel offended, devalued, and violated, the problem with is not the person criticizing you - it's you.
Without people blaspheming God, I never would have woken up to the falsehood of my own fundamentally held beliefs - and yes, they brought me a lot of security and peace. But I was not better off believing a lie. It really is better to have one's own beliefs criticized, examined, and scoured, by rigorous examination, by insult, by satire, by contempt. Sure, it hurts. So does surgery and dentistry.
I don't see blasphemy laws as protecting my dignity. I see them as a condescension, as a statement that I must be protected from painful criticism, and as a threat, as an attempt to control what I am allowed to hear. I'd rather live in a society with a minimum of that, thanks, and I don't think that it's a human right to impose that sort of control on either my tongue or my ears.