I wonder if it is blasphemy to say that God did not do something
when the Biblical text explicitly says He did that very something.
My understanding is Jesus is talking about people witnessing an event, so there
is literally no dispute about it happening the only issue is how and why.
At this point people who choose to lie and deny a work clearly of the Lord are blaspheming the Spirit.
Some interpret this as the hardness of their hearts but I think equally people can do things out of loyalty to a particular group, and are not really thinking it through. The problem is this does not mean it is not a sin.
Now where this issue comes to bear is with people like Benny Hinn. I have testimony about the approach to God he takes, the various herisies he has preached, yet people even here have testified to miracles done at his meetings.
Now I accept the Lord can work anywhere, it is a question of the person calling out to the Lord and the Lord himself. It does not imply the Benny is right or sound. Often in these groups they do not claim to be the source of healing but testifying to the healing after the fact. It would be a bigger issue for me if Benny laid hands on people and they were healed. I have this problem because of testimonies of friends of Benny and others in his ministry who 100% are not of the Lord.
But if the Lord moves and works within these contexts, then amen.
I would caution, taking Jesus's description that the pharisees cast out demons, and said this was the work of God, so how could such people not apportion similar power to his works when they were hypocritical and deceptive people.
This leads me to great caution and openness to how the Lord deals with people and uses every opportunity to bring glory to His name.