It's hard for us modern thinkers, even theologians, to resolve the mystery inherent in "miracle" stories. Actually, the word "miracle" does not appear in the NRSV translation of the Gospels.
Many believers don't give it much thought. The stories say what they say, but deep down, the ways in which Jesus cures "the lame, the maimed, the blind, the mute" (Mat 15:30) are simply vague. From a practical standpoint, the Gospels were written decades after the crucifixion and the eyewitnesses available to the authors may have forgotten a few details.
To the determined unbeliever, the stories are scientific impossibilities, or fabrications. Even John 9's story of the man known to be blind from birth, with corroborating parents and a clearly witnessed cure, is denied. Unbelievers want proof.
The Gospels are the proof. Although we don't have the original, handwritten, first-century texts, we have copies believed to be legitimate copies from the next couple of centuries. The Gospels are largely eyewitness accounts, written some decades after the formation of the first Christian communities. Scholars debate the sources available to the Gospel authors. For example, consider how the authors knew details of the private interrogation of Jesus by Pontius Pilate. Well, Romans were good recorders of history, and there may have been a housekeeper who overheard the events. If my hypothesis is that John 9 is true, my evidence is the Gospel, an eyewitness account of the blindness and the cure.
The non-believer can't deny the Gospels out of hand. No matter how unscientific the texts may be, there is evidence of the events copied from to the first century CE accounts, and little or no comparable evidence to the contrary. Even presuming inaccuracies due to the passage of time before the Gospels were written and due to the messiness of oral accounts, the stories aren't complicated. A blind guy encountered Jesus and was cured. There are dozens of examples and verses claiming that hundreds more occurred.
It is not the responsibility of the believer to provide more proof beyond the eyewitness accounts. Canonical miracles do not need to be explained, although they are mysterious. It's OK that miracles are mysterious, but the stories say what they say. The man was blind and his blindness was cured.
