The best we can do is to answer the question in the context of what Jesus said, and then follow this up with some examples of what it is not.
Jesus explains that Satan cannot cast out Satan, but only one stronger than Satan can do this work. And he cautions His critics that to attribute his power to any other source than the Godhead is to lie against the Holy Spirit.
Don't be too quick to bite this one. Remember, Jesus speaks of those who will say, "Didn't we heal in your name?"
To which Jesus will say, "Be gone, I never knew you."
Which just means that preachers who invoke the scare line of blaspheming the Holy Spirit could be in trouble on Judgment Day.
It is important to realize that questioning miracles performed during healing services is not wrong. We have to look at the evidence. If we are to be faithful stewards, we need to ensure that charlatans are not sucking our money for their own gain. And we need to be careful about what teachings we believe.
A preacher can say a lot of good and true things, but still have an inflated sense of his own ministry. If it sounds like he's telling you to not criticize him, that is indeed what he is doing.
Some churches are effective because they are run by a dominant person. In fact, some of the biggest churches are run by these kind of people.
Some ineffective churches are also run by dominant people.
The difference is the person's ability to sense what is right for their ministry. Which is not the same as being more spiritual. It is just that effective people enlist effective prayer. Prayer can cover a multitude of sins! And we realize that we are not without fault ourselves.
Obviously, not your average person who just rides out life without turning to God. Of course, they don't make it either. But I envision those who see themselves as among the rich and powerful, like the Pharisees of Jesus' day. Now that is what could lead to the blasphemy against the Holy Spirit.
This is probably where we can get a clear picture of this sin. Events like that leave me in awe of God.
But as for someone using the threat of blaspheming the Spirit to get his own way, well, judge for yourself. You can judge a Christian by the sum total of all their fruit, not just by the fact that they do miracles.
And if you are worried that you have blasphemed the Spirit? Your biggest concern is learning to forgive yourself. Stop pretending that you can make yourself good, accept that you cannot survive without crutches, and get up and walk as Jesus gives you strength. Right?
And may He do the work of cleansing and making you whole. May you be
complete in Him.