The word "blaspheme" means "to speak evil of; defame; or revile."  Jesus said that blasphemy against the Holy Ghost is attributing the working of the Holy Spirit to the devil. Many people in the Bible did this, including Saul, who became the Apostle Paul. But in 1 Timothy 1:13, Paul said he received mercy for his blaspnemy because he had done it "ignorantly in unbelief." The blasphemy against the Holy Ghost that Jesus warns about must be the willful reviling of the Holy Ghost with knowledge of what is being done.

Hebrews 6:4-6, qualifications are placed on those who can fall away from grace. This indicates that only a mature Christian can commit such a thing.  Blasphemy against the Holy Ghost, rash statements spoken against the Holy Spirit in ignorance or unbelief by those who don't really know what they are doing can be forgiven.

From our human perspective, no clear line can be drawn as to when someone becomes accountable for blasphemies and has committed this unpardonable sin. We can be assured that God knows the hearts of all men and that He will judge righteously concerning this.  God's Word does show us that when anyone becomes a "reprobate," they lose all conviction from God (Romans 1:28).

Anyone who is convicted and repentant over having possibly blasphemed the Holy Ghost has not yet reached the place where it is unpardonable or they wouldn't care. We must keep our heart tender and sensitive to Him and listen to His voice speaking to us through His Word.