Blasphemy in Scripture is speaking against God with contempt, treating His name or His works as something common or false. Under the Law in Leviticus 24 it carried the death penalty. The Pharisees accused Jesus of it in Mark 2 when He forgave sins. There’s Matthew 12, where Jesus warns that blasphemy against the Holy Spirit won’t be forgiven.
That passage sent me searching years ago. I found Paul calling himself a former blasphemer in 1 Timothy 1:13, and saying he got mercy anyway (see this thread). The unforgivable sin is a settled, hardened rejection of the Spirit who draws us to Christ. I do take His name a lot more seriously than I used to.