Blake's myth has a (at least) double meaning: "It was at once a reading of history and an interpretation of his mystical experiences" (from Percival p 47). The myth is about Albion and about Blake's life.
A thumbnail evaluation suggests that Blake was (and remained) a sensitive social and political animal; his early poems are full of biting social community (look at 'London'). He was also in love with poetry and philsophy of all sorts (including the Bible). Both of these qualities distinguished him from the generality of the population-- of all classesEarly in his life Blake and his wife joined a new church of Swedenborgians, but he soon outgrew that 'system'. He had a contemptuous opinion of organized religion in England from early days and throughout his life.
He read omnivorously and seemed to retain (and use) everything that came into his mind. Some of his most influential reading was the Bible, Plato, the Egyptians, the neoPlatonists, Paracelsus and Boehme. He assembled all this ancient and medieval wisdom (discarded by Western culture with the Enlightenment) into his system, what we call his myth.
Most of the literature and art of special interest to Blake expressed the almost universal concept of metaphysical reality from the days of the earliest Mesopotamians and Egyptians to the present. Blake drew on these symbols everywhere along the spectrum of time.Myths are natural (physical), metaphysical (theological), psychic (psychological), and material (literal); some are a combination. To these we might add the moral or ethical.) (Blake's is definitely a combination of all of them.)
All of these various elements add up to a reality that exists in two spheres:
upper and lower
above and below
Eternity and Time
spiritual and material
Heavenly and Worldly
dry and moist souls
good and evil
But at the deepest level these pairs belong to one another and God is in all ( "as above so below").
These are all ways of looking at the fundamental metaphysical reality of life.
The kernel of meaning in all this wisdom, in all of Blake's system was the myth of the descent of the soul into generation (this fallen world), her extensive travail here and eventual return to the fount of life from which she came.
(The above is an extract from the Blake Primer.