There are two instances in which the gospels tell of Jesus using spit in the eyes of a blind man whom he healed:
Mark 8:23-25: "When they arrived at Bethsaida, they brought to him a
blind man and begged him to touch him. He took the blind man by the
hand and led him outside the village. Putting spittle on his eyes he
laid his hands on him and asked, "Do you see anything?" Looking up he
replied, "I see people looking like trees and walking." Then he laid
hands on his eyes a second time and he saw clearly; his sight was
restored and he could see everything distinctly.
In this instance, Jesus was only partly successful at his first attempt, but succeeded on the second.
John 9:6-7 "When he had said this, he spat on the ground and made clay
with the saliva, and smeared the clay on his eyes, and said to him, Go
wash in the Pool of Siloam. So he went and washed, and came back able
Saliva was widely reported to have medicinal properties in the ancient world. For example, Celsus and Galen mention its healing properties and Pliny collected together many instances of its use in the treatment of boils, pains, sores, snake bites, epilepsy and eye disease. Plinio Prioreschi reports in A History of Medicine: Roman medicine, page 728, that the god told Vespasian to spit in the eyes of a blind man, who would thereby be cured.
The Jews of the second temple period knew of a tradition that the saliva of a legitimate, first born heir would have healing properties against injury or disease. When we look back just a few verses in the context of John 9:6-7, at John 8:8, we read of Jesus proclaiming to be the son of God:
I testify on my behalf and so does the Father who sent me." So they
said to him, "Where is your father?" Jesus answered, "You know neither
me nor my Father. If you knew me, you would know my Father also."
Then in 8:25-29 (with further references by Jesus to God as his father, throughout the remainder of chapter 8):
So they said to him, "Who are you?" Jesus said to them, "What I told
you from the beginning. I have much to say about you in condemnation.
But the one who sent me is true, and what I heard from him I tell the
world." They did not realize that he was speaking to them of the
Father. So Jesus said (to them), "When you lift up the Son of Man,
then you will realize that I AM, and that I do nothing on my own, but
I say only what the Father taught me. The one who sent me is with me.
He has not left me alone, because I always do what is pleasing to
Having told the Pharisees that he was the legitimate son of God, all that remained was for him to demonstrate this by performing something that first-century Jews expected a legitimate, first born heir to do. So, then in verse 9:1: "As he passed by he saw a man blind from birth."
Thus, the significance of Jesus using spit, in the context of John chapters 8 and 9, is the affirmation that he is the only begotten son of God.