First a definition of blasphemy: 
[An] impious utterance or action concerning god or sacred things … irreverent behavior toward anything held sacred, priceless, etc.”
Nobody likes it when what they hold sacred and true is made a subject of contempt or ridicule, and nor can they be expected to; atheists also don’t like it when their beliefs are ridiculed and they are no different from anyone else.
The question though is what to do when it happens, as it surely will from time to time. One way to find an answer is to go back to the original texts and see what they have to say about it, or if the lived example of the Teacher has any bearing on the question.
In Buddhism we don’t have to go very far: the 1st discourse of the 1st book of the discourse collection actually opens with two wanderers from another sect following along behind the Buddha and the monks as they tour northern India.
The two are teacher and disciple and the teacher is speaking in praise of the Triple Gem, while the student is continually speaking in dispraise. The monks get to talk about it, and it comes to the attention of the Buddha. He then gives direct instructions on how to approach the problem, saying: 
“Should others, monks, speak dispraise about me, or the Dhamma, or the Sangha, do not let there be any hatred, resentment, or dissatisfaction for you in your mind, for if you were to be angry or displeased that would be an obstacle for you. If you were to be angry or displeased would you be able to recognise whether what the other said was well-spoken or ill-spoken?”
“Surely not, venerable Sir.”
“Instead you should explain the false as false, (saying): `This is false, this is untrue, there is nothing of this in us, this is not found amongst us.’
Should others, monks, speak praise about me, or the Dhamma, or the Sangha, do not let there be any joy, pleasure, or elation for you in your mind. If you were to be joyful, pleased or elated that would be an obstacle for you. Instead you should explain the true as true, (saying): `This is so, this is true, there is this in us, this is found amongst us.’ ”
Both hatred and joy are inappropriate responses because they get in the way of seeing the truth. It may be that what is said is wrong, misinformed, culumnous or wilful, but the way to deal with it is not through anger, but through quiet, but firm, assertion of the truth.
Similarly what is said might be through faith, conviction or zeal, but still it should be tested against the truth, and if it is true then it can be acknowledged as so. Not everything a person says through faith may be true however, then reasoned correction is again appropriate.
I cannot help but think that if everybody had an enlightened teacher we would find ourselves living in a much more peaceful world today.
from Shwezigon Pagoda, Bagan, Myanmar