Blasphemy of the Holy Spirit is also called the unpardonable or unforgivable sin. See Unforgivable Sin for more discussion on the forgiveness aspect of this. Blasphemy of the Holy Spirit is found in Matthew 12:31-32 and parallel passages in the gospels.
“And so I tell you, every sin and blasphemy will be forgiven men, but the 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.”
The context of this passage is that Jesus was performin miracles and casting out demons. The Pharisees stated that Jesus was using the power of Satan to do such acts. Attributing the work of the Holy Spirit to that of Satan is what Jesus calls blasphemy against the Spirit.
This sin is not an accident or even an intentional sin. Instead it is reflective of the attitudes of the Pharisees. They were so set against the work of Jesus that they were willing to accept that Satan was working through Him rather than the Holy Spirit. While there are many sinful people in the world today, few are so far from God as to attribute the power of God as Satanic.
Consider that many people accept that the universe was created in a Big Bang and that man is a product of millions of years of evolution. God has been taken out of the picture because for humanity to have a Creator means that humanity is subject to that Creator. Sinful people have done a lot to discredit God from having anything to do with our existence. This doesn’t go nearly as far as blasphemy of the Holy Spirit. Blasphemy of the Holy Spirit would have to say that not only were we not created by God but because there is so much suffering in evil in the world that the universe was created by Satan. This is how far removed from reality blasphemy of the Holy Spirit is.
While there are many people with hard hearts today, very few would go so far as to attribute what God is doing as the work of Satan. Those who do so do not want to acknowledge God in any way and are completely lost.