Blench is a verb, so it is something that you must do, just as a person would "talk," "read," or "type."
To "blench" means to "flinch" or, put simply, to react to something unpleasant by drawing back. If someone were to attempt to punch someone else, their target might "blench" and step back.
So, to use the word in a sentence:
My father blenched when my mother hit him.