How to cure "if I blamed someone who we should not blame"?
There's no completely escaping the consequences of a bad deed unless the Karma becomes defunct. But you can do much to mitigate the effects.


If the monk is alive, apologize in person. If he is dead, think of him and apologize.
Do good deeds. ex: offering alms to the monk you insulted or to the Sagha as a whole.
Do meditation. ex: Spreading Metta to all beings, practicing the four foundations of mindfulness etc.


All of the above will help a great deal, but only a Buddha would know what exactly you should do to make a Karma defunct, if it is possible.