I would like to suggest another, no not another, but a companion reason for Blasphemy against the Holy Spirit to be unforgivable, considering that Blasphemy against Jesus is forgivable, according to Luke 12:10. Jesus has power over the physical bodies of all men, but I haven't seen anywhere that the Holy Spirit has any power other than the ability to help believers in various ways. Therefore, to give the Holy Spirit power over sinners to "protect' it's dignity, God made it a deadly sin to defame it. If defame is not what everyone feels blasphemy might be, then you may insert your own word.