I get it from the context of the passage Matthew 12:22-32 which speaks of this. This is when you call what is good evil. That is what the Pharisees were doing when Jesus said this to them. It is a blasphemy against the Holy Spirit because you are speaking evil against the true source of inspiration for those actions.
Of course I believe that sins are self-destructive habits not rule breaking or rebellion. I think the latter definitions have more to do with making religion a tool of power. I also don’t believe God has such a hard time forgiving people that he needs some super spell in order to accomplish it. So I don’t think salvation is about forgiveness or acquiring a get-of-jail-free card, but about changing our habits from the things which are self-destruct to those which build us up.
So the original or first sin in the Biblical narrative is A&E playing this blame game when God confronts them about breaking the commandment. We learn from making mistakes but how can we learn from our mistakes if we refuse to acknowledge them. What can we say then to this willingness on the part of the Pharisees to use rhetoric to label good things as evil? How can anything be more self-destructive than destroying your own moral compass in such a manner – repudiating the dictates of your own conscience. This is definitely a step in the wrong direction from the sin of A&E, not simply failing to acknowledge your mistakes but refusing to even see what is good and evil at all.
I don’t think this is so much about some black and white rule as about there being a point of no return in the transition from good to evil. If we start lying about what is good and evil, warping our own conscience by telling ourselves that those doing good are evil and that evil things are good, then I fear we lose what tenuous connection we have with God altogether and there is no coming back from that.