A symbol is not what it symbolises. It is serious error to confuse the reality with the symbol which communicates or mediates it. Blake was cautious about allowing symbols to replace the reality they stood for.
The ark of the temple is a case in point. As a primary symbol for the Jews of the presence of God, it became an object of worship itself. Furthermore the worshippers were kept at a distance from the ark which was hidden within veils. Above the ark were positioned two golden cherubim who covered it with their wings. The symbolic meaning of the ark and the cherubim involved the law of God and his mercy as the physical manifestation of his spiritual presence. The physical artifacs became a covering for the numinous. But could the presence of God be localised in these objects which became central to the practice of Hebrew religion?
So we have in the book of Ezekiel, the covering cherub becoming a symbol for the unrighteousness which claims the position of God. Because the prince of Tyrus, blessed with wisdom, wealth and power, claims to be God he is brought down to the depths. He is called the 'covering cherub' as one who, selected by God, covers the Divine presence with his own Selfhood.
 The word of the LORD came again unto me, saying,
 Son of man, say unto the prince of Tyrus, Thus saith the Lord GOD; Because thine heart is lifted up, and thou hast said, I am a God, I sit in the seat of God, in the midst of the seas; yet thou art a man, and not God, though thou set thine heart as the heart of God:
 Thou hast been in Eden the garden of God; every precious stone was thy covering, the sardius, topaz, and the diamond, the beryl, the onyx, and the jasper, the sapphire, the emerald, and the carbuncle, and gold: the workmanship of thy tabrets and of thy pipes was prepared in thee in the day that thou wast created.
 Thou art the anointed cherub that covereth; and I have set thee so: thou wast upon the holy mountain of God; thou hast walked up and down in the midst of the stones of fire.
 Thou wast perfect in thy ways from the day that thou wast created, till iniquity was found in thee.
 By the multitude of thy merchandise they have filled the midst of thee with violence, and thou hast sinned: therefore I will cast thee as profane out of the mountain of God: and I will destroy thee, O covering cherub, from the midst of the stones of fire.
In Milton the Covering Cherub, the Spectre of Albion, sinking down into the Death of Rome, Babylon and Tyre descends into Generation as Satan or Urizen.
Milton, Plate 9, (E 103)
"Rintrah rear'd up walls of rocks and pourd rivers & moats
Of fire round the walls: columns of fire guard around
Between Satan and Palamabron in the terrible darkness.
And Satan not having the Science of Wrath, but only of Pity:
Rent them asunder, and wrath was left to wrath, & pity to pity.
He sunk down a dreadful Death, unlike the slumbers of Beulah
The Separation was terrible: the Dead was repos'd on his Couch
Beneath the Couch of Albion, on the seven mou[n]tains of Rome
In the whole place of the Covering Cherub, Rome Babylon & Tyre.
His Spectre raging furious descended into its Space
PLATE 10 
Then Los & Enitharmon knew that Satan is Urizen
Drawn down by Orc & the Shadowy Female into Generation"
Milton, PLATE 37 , (E 138)
"All these are seen in Miltons Shadow who is the Covering Cherub
The Spectre of Albion in which the Spectre of Luvah inhabits
In the Newtonian Voids between the Substances of Creation
The Heavens are the Cherub, the Twelve Gods are Satan"
Orc begs Jerusalem not to assume the Human Form by weaving Satan for a Covering.
Milton, PLATE 18 , (E 111)
"Orc answerd. Take not the Human Form O loveliest. Take not
Terror upon thee! Behold how I am & tremble lest thou also
Consume in my Consummation; but thou maist take a Form
Female & lovely, that cannot consume in Mans consummation
Wherefore dost thou Create & Weave this Satan for a Covering[?]
When thou attemptest to put on the Human Form, my wrath
Burns to the top of heaven against thee in Jealousy & Fear.
Then I rend thee asunder, then I howl over thy clay & ashes
When wilt thou put on the Female Form as in times of old
With a Garment of Pity & Compassion like the Garment of God
His garments are long sufferings for the Children of Men
Jerusalem is his Garment & not thy Covering Cherub O lovely
Shadow of my delight who wanderest seeking for the prey."
Jerusalem, PLATE 89, (E 247)
"Thus was the Covering Cherub reveald majestic image
Of Selfhood, Body put off, the Antichrist accursed
Coverd with precious stones, a Human Dragon terrible
And bright, stretchd over Europe & Asia gorgeous
In three nights he devourd the rejected corse of death
His Head dark, deadly, in its Brain incloses a reflexion
Of Eden all perverted;"
Image from Rosenbach Museum, Philadelphia
Courtesy of Wikipedia
The Covering Cherub is the image which absorbs all the error and false perceptions which prevent man from understanding that reentry into Eden is possible.