Dramatic irony occurs when readers know more about a situation than a character does. Why is John struck by Hale’s declaration that “some secret blasphemy” (lines 946-947) has caused all of the confusion?

The secret blasphemy, as far as the Puritans are concerned, is the dark secret that John keeps hidden away. He and Abigail Williams, their young maid, had been hitting the sheets. The affair has been plaguing John's conscience. Hale intended his declaration towards witchcraft but John feels it directed to himself. The dramatic irony is that Hale, at this point, does not know about John and Abigail's "play time" activities. I hope this sounds clear enough.