First we must define what is blasphemy against the Holy Spirit. The Catechism states:
“Therefore I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven.” There are no limits to the mercy of God, but anyone who deliberately refuses to accept his mercy by repenting, rejects the forgiveness of his sins and the salvation offered by the Holy Spirit. Such hardness of heart can lead to final impenitence and eternal loss (CCC 1864).
Blasphemy against the Holy Spirit is to refuse God’s mercy. It is a refusal to repent of sin. God does not bring anyone into his kingdom against his/her will. Human persons have the ability to reject God’s mercy and refuse forgiveness of sins which leads to eternal separation from God.