There is no such thing as " Sin" as it is defined in the Bible, so who gives a s**t what their unforgivable sin is in reality ? it won't affect your eternal future.
Having said that, I have never seen a satisfactory explanation of what exactly was in the Bible Writer's mind when he put this drivel down. Every attempt at exegesis on this strikes me as mere personal opinion on the part of the Exegete, who has no real reason for said opinion.