THE TWO CONTRARIES

Blake wrote extensively about Contraries in Plate 3 of The Marriage of Heaven and Earth:

"Without Contraries is no progression.
Attraction and Repulsion, Reason and Energy.
Love and Hate are necessary to Human existence.
From these contraries spring what the relgion call Good and Evil.
Good is the Passive that obeys Reason.
Evil is the Active springing from Energy.
Good is Heaven; Evil is Hell."
Also Angels and Devils.

In MHH Blake mercilessly parodied the conventional images of angels, angelic, etc.
He chose to be a devil (a nonconformist, a revolutionary, call him what you will). He said that Milton (one of his maximum heroes) belonged to the devil's party without knowing it (MHH 6).

He wrote the first of the 'major prophecies' and called it Milton: he brought Milton back from Heaven to complete the work he had left unfinished.

In his early works Blake labored with good and evil, sin and faith, Heaven and Hell and many such Contraries. He focused on the good and evil of God, and most of it was on the evil side. (Here I'm using 'evil' in the proper sense rather than the ironic sense of MHH.)

At four He spoke of the angry God at the window; he spent many years denouncing and defying that God. But eventually the Contrary became Father and Son (thump on the head and healing balm).
Satan, who had been his hero as a denouncer of Old Nobodaddy, ceased to be a person and became a state (we're all capable of being in the state of Satan. It's the Selfhood,the Spectre.)

The idea of Contraries stems from the fact that the corporeal mind thinks in terms of dualism, and Blake like all of us was condemned to dualistic thinking. As he said in MHH, it's "necessary to Human existence".

But Blake's true Heaven, like that of Jesus (in John 17) was no longer dualistic; it was All.