Speaking a word against the "Son of Man" (emphasis on humanity) is NOT blasphemy of the Holy Spirit.
I don't think that what the Jews who suggested that Jesus was casting out demons by power that he got from Hell itself was all that Jesus was talking about when he spoke that particular word of truth. Because to call the Christ a "prophet of God" is an understatement. But that does not mean that Jesus did not speak prophetically. Indeed He did. Part of what we read about in the Scripture regarding the unpardonable sin includes a prophetic utterance. Jesus was speaking about something yet to come. Something that would have the smell of what the Jews of His day were doing but something that could NOT be counted as blasphemy against the Son of Man.
It's possible that our Lord looked upon the Children of Israel with such grace so as to gently admonish them against continuing down that path for there lies nothing but ruin. Jesus knew they were talking of Him and that they really didn't like the things that He was saying. And He knew they went too far when they suggested that he had an unclean spirit.
I'm thinking that Jesus wasn't delivering a scathing rebuke but instead was warning them off from something. When the source of evil-speaking against Christ is ignorance, misconception, or ill-informed prejudice, then that blasphemy is as pardonable as any sin. They could later repent of their careless neglect of His work or their mistaken opposition to it, and when they did repent, they were forgiven. Perhaps some of the very men who said such things were also present on the day of Pentecost later. Maybe they were part of the 3,000 that were added to the kingdom as they listened to Peter preach with their hearts burning inside of them. I'm unsure. The Bible is silent on that and I guess what I'm trying to say is that I am not their judge but I can indeed conceive of circumstances where they too could be saved. That's up to Jesus and not me. It's perfectly acceptable for me to say, "Duh, I dunno," when in fact I really do not know.
Also, there will come a time when men will revile Christians even more so than they do today. We are told of a time of tribulation where such things that have never happened before, nor shall happen again, occur. We are even given specific instructions about such things. For instance, there will come a time (and now is) where loved ones will betray those who believe. Jesus told us about this in advance so that when it happens we will not be discouraged. Those who search the Scripture daily and go before the Lord in inquiry, who partake of the Gift of the Holy Spirit (our Teacher) shall not be taken by surprise. One of the reasons that we are given so much information regarding the wrap-up of the age is so that our enemy can not prevail. Yet there will come a time where even the elect (should such a thing be possible) were deceived. Part of that deception is the calling of GOOD to be evil and the calling of EVIL to be good.
There are other prophecies that have been spoken so that when they are seen by His followers the end result will be an increase in belief and trust, for in truth, they were spoken of thousands of years prior. Blasphemy of the Holy Spirit, the sin that shall never be pardoned, is one of those foretold from ages past. I don't think we fully understand what is meant because we've not yet come to the ultimate culmination of the age.