UNVEILED

A dissertation, Revolution & Revelation: William Blake and the Moral Law, by Michael Farrell of the University of Oxford clarifies Blake's use of the Bible in developing the style and content of his writing. In this paragraph it becomes clear that Blake's use of Vala's veil had antecedents in Biblical accounts:

 "2 Corinthians 3 is instructive in analysing Blake’s conception of the revolutionary Christ. The oppressive Law of the Old Testament is essentially abolished by Christ and the new Law which he establishes 'consists not of a written law but of the Spirit' (2 Cor. 3: 6). The Law of Elohim the comminatory God of Judgement is supplanted by the Law of the Spirit the merciful Law of Jehovah which operates not in an external and abstract system of moral codes but internally in and through the Spirit. The old covenant is effectively a veil separating the individual from the Spirit: 'The veil is moved only when the person is joined to Christ' (2 Cor. 3: 14). For Blake the epiphany of the Christ at the apocalypse is the ultimate revelation or unveiling of Error: the word 'revelation', etymologically, derives from the Old French reveler or Latin revelare, from re, meaning 'again', in the sense of a reversal, and velum, meaning 'veil'. The word revelation has etymological links with the word 'apocalypse', which, deriving from the Greek apokaluptein , also means to unveil or reveal. Thus in Blake’s secular apocalypse the individual must necessarily undergo a Last Judgement in casting off Error, or the old Law, and embracing Truth, the covenant or law of the Spirit." (Page 7) Creative Commons License 3.0
 
RSV
2 Corinthians 3
[2] You yourselves are our letter of recommendation, written on your hearts, to be known and read by all men;
[3] and you show that you are a letter from Christ delivered by us, written not with ink but with the Spirit of the living God, not on tablets of stone but on tablets of human hearts.
[4] Such is the confidence that we have through Christ toward God.
[5] Not that we are competent of ourselves to claim anything as coming from us; our competence is from God,
[6] who has made us competent to be ministers of a new covenant, not in a written code but in the Spirit; for the written code kills, but the Spirit gives life.
[7] Now if the dispensation of death, carved in letters on stone, came with such splendor that the Israelites could not look at Moses' face because of its brightness, fading as this was,
[8] will not the dispensation of the Spirit be attended with greater splendor?
[9] For if there was splendor in the dispensation of condemnation, the dispensation of righteousness must far exceed it in splendor.
[10] Indeed, in this case, what once had splendor has come to have no splendor at all, because of the splendor that surpasses it.
[11] For if what faded away came with splendor, what is permanent must have much more splendor.
[12] Since we have such a hope, we are very bold,
[13] not like Moses, who put a veil over his face so that the Israelites might not see the end of the fading splendor.
[14] But their minds were hardened; for to this day, when they read the old covenant, that same veil remains unlifted, because only through Christ is it taken away.
[15] Yes, to this day whenever Moses is read a veil lies over their minds;
[16] but when a man turns to the Lord the veil is removed.
[17] Now the Lord is the Spirit, and where the Spirit of the Lord is, there is freedom.
[18] And we all, with unveiled face, beholding the glory of the Lord, are being changed into his likeness from one degree of glory to another; for this comes from the Lord who is the Spirit.

Yale Center  for British Art Blake's Watercolours for the
Poems of Thomas Gray

With the help of Michael Farrell's explanation of Blake's use of the old and new covenants, and the passage he cites in Second Corinthians we can see more of Blake meaning in the following passage. Jerusalem represents the new covenant in which the law is expressed through the indwelling spirit. Albion has not been able to separate himself from dependance on the letter of the law as an external code which hides the face of God behind a veil of separation. Although the veil was removed by Christ, Albion has not freed himself from the guilt which causes him to use the veil as a 'law, a curse and a terror' against his children.
 
In his bitterness, Albion's dying curse is that Jerusalem, his own spiritual nature, be drawn 'down into this Abyss of sorrow and torture.'    

Jerusalem, Plate 23, (E 168)
"Jerusalem! Jerusalem! deluding shadow of Albion!
Daughter of my phantasy! unlawful pleasure! Albions curse!
I came here with intention to annihilate thee! But
My soul is melted away, inwoven within the Veil
Hast thou again knitted the Veil of Vala, which I for thee       
Pitying rent in ancient times. I see it whole and more
Perfect, and shining with beauty! But thou! O wretched Father!

Jerusalem reply'd, like a voice heard from a sepulcher:
Father! once piteous! Is Pity. a Sin? Embalm'd in Vala's bosom
In an Eternal Death for. Albions sake, our best beloved.         
Thou art my Father & my Brother: Why hast thou hidden me,
Remote from the divine Vision: my Lord and Saviour.

Trembling stood Albion at her words in jealous dark despair:
He felt that Love and Pity are the same; a soft repose! 
Inward complacency of Soul: a Self-annihilation!                 

I have erred! I am ashamed! and will never return more:
I have taught my children sacrifices of cruelty: what shall I answer?
I will hide it from Eternals! I will give myself for my Children!
Which way soever I turn, I behold Humanity and Pity!

He recoil'd: he rush'd outwards; he bore the Veil whole away     
His fires redound from his Dragon Altars in Errors returning.
He drew the Veil of Moral Virtue, woven for Cruel Laws,
And cast it into the Atlantic Deep, to catch the Souls of the Dead.
He stood between the Palm tree & the Oak of weeping
Which stand upon the edge of Beulah; and there Albion sunk       
Down in sick pallid languor! These were his last words, relapsing!
Hoarse from his rocks, from caverns of Derbyshire & Wales
And Scotland, utter'd from the Circumference into Eternity.

Blasphemous Sons of Feminine delusion! God in the dreary Void
Dwells from Eternity, wide separated from the Human Soul         

But thou deluding Image by whom imbu'd the Veil I rent
Lo here is Valas Veil whole, for a Law, a Terror  & a Curse!
And therefore God takes vengeance on me: from my clay-cold bosom
My children wander trembling victims of his Moral justice.

His snows fall on me and cover me, while in the Veil I fold    
My dying limbs. Therefore O Manhood, if thou art aught
But a meer Phantasy, hear dying Albions Curse!
May God who dwells in this dark Ulro & voidness, vengeance take,
And draw thee down into this Abyss of sorrow and torture,
Like me thy Victim. O that Death & Annihilation were the same!"