I think some light might be shed on the issue surrounding blasphemy against the Holy Spirit by looking at the Donatist controversy. That controversy concerned whether Christians who had denied their faith to avoid martyrdom could rejoin the Church once Constantine had become Emperor and public worship was now legalized in the Roman Empire. After all, where were these guys when the Church needed them to stand up for their faith? They chickened out and denied it in public, right? The Donatists said that they couldn't rejoin the Church, but that position was actually condemned.
Apparently renouncing or deny Christ (or even the Holy Spirit) was not sufficient to remove them from the possibility of receiving God's grace, according to the Church at that time.