The sin is defined as attributing that of God to the devil. But from what I understand about the circumstances, the Pharisees were right there all along when Jesus was performing his miracles, and it seemed like it was pretty convincing, but the Pharisees had such hard hearts towards anything against their system of theology that they refused to even see Jesus as the son of God, despite all the miracles. They should have seen it, but instead blamed it on satan. Plus it seems logical that the most hardened would take the longest time to be forgiven due to extreme stubborness.
But what I wondering about attributing something of God to the Devil mean you mistaked something Godly for being demonic, due to limited knowledge. Problem is, I have accused many things of being demonic, like various Marian Apparitions, the Government, the Current Church system, illness, annoyances from relatives, ect. Mostly because they yeilded bad fruits, and not because of any pre-concieved notion.
The best explanation I have heard was a willful, and knowledgeable acts of attributing acts of the Holy Spirit with that of the devil. This would be someone who hates God and is trying to slander the Holy Spirit.