The account in the Gospel of Mark appears to explain it:
And the scribes that came down from Jerusalem said, He hath Beelzebub, and, By the prince of the demons casteth he out the demons. And he called them unto him, and said unto them in parables, How can Satan cast out Satan? And if a kingdom be divided against itself, that kingdom cannot stand. And if a house be divided against itself, that house will not be able to stand. And if Satan hath risen up against himself, and is divided, he cannot stand, but hath an end. But no one can enter into the house of the strong man, and spoil his goods, except he first bind the strong man; and then he will spoil his house. Verily I say unto you, All their sins shall be forgiven unto the sons of men, and their blasphemies wherewith soever they shall blaspheme: but whosoever shall blaspheme against the Holy Spirit hath never forgiveness, but is guilty of an eternal sin: because they said, He hath an unclean spirit. (Mark 3:22-30 ASV)This seems rather plain. Jesus did have a spirit—the Holy Spirit. By saying "he hath an unclean spirit" they were rejecting the Holy Spirit, labeling Him an evil spirit. It says that Jesus taught about "the unforgivable sin" BECAUSE they had said this. The clear implication is that this act of speaking against the Holy Spirit as an evil spirit WAS blasphemy against Him. This also fits well with the definition of "blasphemeo"—to speak evil of, to revile.
Being unfamiliar with your terminology, I'm not sure how closely this view would line up with "final impenitence."