Firstly, I'll gloss over the blasphemy law itself. But I'd rather take this thread as an opportunity to briefly discuss the whole constitutions as opposed to this one minor law. There are probably a few different positions one can take with regards to this law. I personally am not in favour of policing morality, and especially not making immorality punishable by such harsh punitive measures.
Even so, the first level at which one might criticise the current situation (and not specifically the law), is one I think we can all agree on. Mob rule is never legitimate, and any case of blasphemy where due process of law has been usurped is unacceptable. The second position is probably regarding the current form of the law, and any issues of jurisprudence regarding blasphemy. We can debate about whether a witness ought to be sufficient, about how other motives might drive a conviction, targeting of minorities, and the severity of it. I'd go as far as this second position. The last position is those who are totally opposed to any blasphemy law, policing of morality, against forms of protection of religious sentiment and advocates of a high degree of freedom of expression. I personally don't take this stance for Pakistan, it is an Islamic Republic, and this position is detached from the current reality of Pakistan.
However, with that said, blasphemy law is only a minor issue that is related to larger problems we face. There are FAR more pressing matters that are related to the blasphemy law that we should focus our efforts on. Mob rule in enforcement of blasphemy is the result of a lack of law order, an uneducated society etc. The existence of certain aspects of the current constitution are questionable. In my opinion, there ought to be a new constitution that should be developed, or at least this one requires major reform.
Reform isn't easy of course, and a new constitution is even harder. The first and hardest step would be to consolidate enough political credibility. Which ironically, in my opinion was lacking when this constitution was drawn up in the first. And even after it was drawn up, it became a political system misused by Zia, I take as much issue to radicalism he placed therein as I take issue with what he did it for. Zia, didn't use Islam at a state level because he was a pious man, he did it to legitimise himself and for political power. That's only one aspect to consider though.