Blaspheme is when you call that which is of God of the Devil.
Verily I say unto you, All sins shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme: But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation: Because they said, He hath an unclean spirit.
-Mark 3:28-30
"They" in this context are the scribes, Pharisees, chief priests, and elders who, like church building leadership today, have tasted of the sweet things of the Holy Spirit of God, but turns around and calls Him (i.e. the Holy Spirit) of the Devil (i.e. an unclean spirit) when the truth is preached unto them (like when Steven did so in Acts 7, if you are listening to our audio downloads each week, we just finished covering that), and those things which are not the Holy Spirit of God, those things that are unclean, are said by these wicked men to be the Holy Spirit.
This is why I find those who are in the so-called "Speaking in Tongues" (a.k.a. Satanic gibberish) movements in such danger of hell fire. They are claiming that which is unclean to be the Holy Spirit of God, and I fear the Living God too much to do such a thing. All blasphemies against God and Christ are forgiven with the repentance (i.e. grief and godly sorrow of wrongdoing) and grace through the blood of Christ, but God does not give remission of sins to those who call the Holy Spirit of God of the Devil because once they get to that point, they are so far gone, they will never be saved because their hearts have become too prideful and corrupt, and there is no chance for them to be humbled to repentance.