I spoke to my father-in-law, a world-reknowned theologian about this. The most important thing that can be said is that no one is sure. So let us be do our best not to blaspheme! And that may be exactly why the verse is in the Bible, to caution us all against claiming to be certain to know God's truth. In fact, claiming to be certain about God's truth may be blasphemy against the Holy Spirit; it is certainly violationc of the First Commandment, for we are setting up our own knowledge and worshipping ourselves, rather than worshipping God.
One common interpretation is that Jesus is saying that the key issue is not what a person says about Jesus himself being the Son of God, but rather whether a person is belittling or trivializing the basic Goodness of God. This would fit with many of Jesus' other teachings in the synoptic Gospels (Matthew, Mark, Luke), where the key is to pray to God and love God "our Father," (Jesus' father, and ours as well) and show that love by our actions to all. In this view, those who refuse to live in this basic state of loving our friends and enemies as ourselves are denying the Holy Spirit and blaspheming against the Holy Spirit, as long as we remain in this resolute commitment to be against love and deny God's Goodness.