Blake was very conversant with what the Bible has to say about Good and Evil:
Gen 1:31 "God saw all that he had made, and it was very good. And there was evening, and there was morning—the sixth day." It seems that everything was very good; there's no polarity here.;
But in Gen 2:8-9 we come to a complication:
"And the LORD God planted a garden eastward in Eden; and there he put the man whom he had formed.
And out of the ground made the LORD God to grow every tree that is pleasant to the sight, and good for food; the tree of life also in the midst of the garden, and the tree of knowledge of good and evil."
This seems to infer that Good and Evil came into existence as a consequence of the (biblical) Fall. In the pristine Garden before Adam and Eve's fatal mistake Evil had not entered the picture. (Some Bible scholars have concluded that the 'fatal mistake' was a culpa felix (Augustine, Aquinas, Ambrose). However it's generally understood as the cause of all unhappiness in our poor World. Imagine how it would be if the 'fatal mistake' had not occurred. Would we be more like the animals? or the angels?
With The Marriage of Heaven and Hell Blake put an entirely new slant on the subject. (The cavalier way Blake used the biblical Fall here illustrates the use Blake put to the Bible in general: like any other document everything was grist for his mill.) Speaking ironically he described Good as being sheeplike and Evil as being active and creative. He described good people as the Elect and active, creative people as Reprobate (btw he included Jesus among the Reprobates- following Isaiah 53:12).
The Elect were the angels in MHH; the Reprobates were the devils.
But Blake didn't stick to these definitions; MHH was the work of an angry young man. The mature Blake returned to more conventional meanings for 'angel' and 'devil'.
Returning to the Tree of the Knowledge of Good and Evil we may read in Blake's Songs of Innocence and Experience:
The Human Abstract.
Pity would be no more,
If we did not make somebody Poor:
And Mercy no more could be,
If all were as happy as we;
And mutual fear brings peace;
Till the selfish loves increase.
Then Cruelty knits a snare,
And spreads his baits with care.
He sits down with holy fears,
And waters the ground with tears:
Then Humility takes its root
Underneath his foot.
Soon spreads the dismal shade
Of Mystery over his head;
And the Catterpiller and Fly,
Feed on the Mystery.
And it bears the fruit of Deceit,
Ruddy and sweet to eat;
And the Raven his nest has made
In its thickest shade.
The Gods of the earth and sea,
Sought thro' Nature to find this Tree
But their search was all in vain:
There grows one in the Human Brain"
(Erdman p. 27)
We may see here the origin of the Tree of Mystery, which in Blake corresponds to the Tree of the Knowlede of Good and Evil; Blake has tried to explain the meaning of the Tree he had read about in Genesis.
Good and Evil are a polarity, and a contrary of the pristine oneness of the original Garden. We may see it as the first contrary, from which all others sprang. We live in a dualistic world, and people in general can only see things in black and white (like infants do). To perceive things as a spectrum, such as 'Good, less good, still less good,' etc. is a step away from the fatal tree, but still a long way from the primeval oneness from which we came and to which we are destined to return.