People relate to and address God in a variety of ways:
1. The most common way is probably profanely, just using God terms as expletives, such as Oh my God or Jesus Christ. I don't think you'll find that form of address in Blake's poetry.
2. In formal worship, such as "Our Father which art in Heaven".
3. In a familiar address such as "God, why did you let that happen?"
4. In the third person such as the four year old Blake describing a personal experience of seeing an angry God in his window, or
5. In personal prayer.
Another way to approach Blake's prayer life is with the general types of Prayer as Praise, Thanksgiving, Confession, Intercession Petition.
To get a handle on Blake's prayer life you might put God, Saviour, Jesus, Christ, Father in the Concordance. Here are a few interesting occurrences:
In the Four Zoas the Saviour is mentioned several times, several times in Milton and many times in Jerusalem. In the letters he is addressed in:
#26 to Butts (April 03; E728)"
"Saviour & I am also grateful to the kind hand that endeavours to lift me out of despondency even if it lifts me too high--"
Reading this letter it seems obvious that Butts' encouragement had brought out the best in Blake, a strong faith that had been overlaid by so much worldly pressure from his supposed sponsor, Hayley.
Butts appears to be Blake's confessor, the one person in his acquaintance that he could express fully his true self.
With Hayley no longer breathing over him Blake found it easy to be civil and appreciative, as we can see in Letter #34 (Jan 04 E739):
"God our Saviour watch over you & preserve you".
By 1805 he had gained enough emotional distance from Hayley even to instruct him in the value of spirituality:
#61 Dec. 1805; Erdman 767):
....I throw Myself & all that I have on our..
Saviours Divine Providence. O What Wonders are the Children of Men! Would to God that they would Consider it That they would Consider their Spiritual Life Regardless of that faint Shadow Calld Natural Life. & that they would Promote Each others Spiritual Labours."
In this passage we see another form of address:
"Would to God.." One of these: ("Would to God that all the Lord's people were prophets") is actually a paraphrase of Numbers 11:29.