At the time that John wrote his Gospel the revolution was over.
Christianity had spread to the Roman world beyond Israel 
and faced the challenge of other religions
and different views of divinity.

As with the whole of John's Gospel, literal veracity
is replaced by adapting the truth to fit the theory.
Honesty is subject to the teacher's need;
as it is in many political speeches
of our own times, today.

The initial subject is the relationship between sickness and sin.
This is developed into whether good works can be done on Sunday,
and runs through to verse 21, including conflict with Pharisees
and a declaration of Jesus as the Son of God.

First Jesus is used to deny any relationship between 
the sins of the parents and the disability of the child,  
which was a common belief in some religious sects of those days.
This story is then mixed with that of Mark at page 33,
with which it shares many features.

Why was the child born blind?
John sees the cause as being to allow Jesus to heal him.
It is a strange theology, placing all physical disability
as a vehicle for divine intervention.1

The pool at Siloam is another different feature in the story.
The pool was in distant Jerusalem and a place of pilgrimage.
It was built to provide water for the city during any siege.
By the time this story was written, the pool had been destroyed,
but the memory of its sacramental use seemingly lived on.

A lot of words are used to provide local evidence for the healing,
leading up to the conflict with the Pharisees over healing on the Sabbath,
which was obviously a bone of contention at the time of writing.

It would seem that John has taken a story from the earlier gospels
and adapted it to his own purposes, adding erroneous detail as needed. 
These are, however, not fairy stories as we might perceive,
but teaching vehicles for his time.