There is only one verse in the bible that might contribute to the belief that blasphemy is unforgivable:
Matthew 12:31-32 (NIV 2010) wrote:And so I tell you, every kind of sin and slander can be forgiven, but blasphemy against the Spirit will not be forgiven. Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.
Honestly I am not entirely sure what this means, and Jesus doesn't seem to have given any further explanation after he says this, but most commentaries I've read think that "blasphemy against the spirit" means a continual disbelief in God. Since it is by believing that you are saved, and not by doing good things (or avoiding bad things).
Actually, maybe the next paragraph does explain this more:
[quote="Matthew 12:33-37 (NIV 2010)"]“]So going by this passage, if you are saved by confessing your faith in Jesus, then you are condemned by denying it.
Wait that can't be quite it, because Jesus says himself that slandering the Son of Man (meaning Jesus) is not unforgivable. We even have an example from Peter, who adamantly denied Jesus in his hour of suffering, and yet was forgiven after Jesus rose from the dead.
So what I think is that it isn't that there's an arbitrary rule that blaspheming against the Spirit is unforgivable, it's because of the way forgiveness and faith work. God's grace and forgiveness is part of God's spirit, so how can you be forgiven if you do not believe in the spirit that is forgiving you?
Or taken another way, if you say that God is evil, when God is the very definition of good, then you are saying that good is evil, and then you're just being illogical.
I honestly don't know if I am thinking about this the right way, so don't take my words at face value. I don't think it's terribly important that we figure out the meaning behind this passage. And I think it's safe to live under the assumption that all sins can be forgiven if you trust God. Don't think you can magically become unsaved forever just by saying something dumb.