Reviewed in The Guardian:
Marsha Schuchard has found that grail of researchers - original documents that confirm suspicions about her subject. In this case they are surviving records of the unworldly Moravian Chapel in Fetter Lane showing how William Blake's family were worshippers at this shrine of eroticism. The Moravians raised prayers to the side-hole of Jesus, the spear wound which was depicted with a vulval shape and was the subject of hymns and ecstasies. "At the love feast our Saviour was pleased to make me Suck his wounds," the woman here identified as Blake's mother wrote in the documents Schuchard has uncovered. [...] The head of the chapel, Count Zinzendorf, had a pioneering policy of sex-education for young marrieds, including texts to recite during climaxes, such as "when my dear husband lets his oil sizzle in me, this grace is a sacrament". Zinzendorf argued that the Old Testament commandment against adultery was out of date because it was based on a time when polygamy was common. He himself had an open relationship with a 14-year-old girl whom he made an eldress of the church. [...] Blake, Schuchard says, learned such manoeuvres as withholding orgasm in order to retain seminal fluid to nourish the brain, putting his wife under the pressure of "enabling her husband to achieve the prolonged erection necessary to the visionary process". It is clearly supposition that Mrs Blake was so beleaguered, though a man who wrote "The lust of the goat is the bounty of God" doesn't seem the sort to compromise on his conjugals. Schuchard also traces Blake's journey through the mystical underground of the supposedly "enlightened" 18th century, a world rich in knowledge of the Jewish Kabbalah, Muslim sensuality, Tantric yogis and Chinese love-texts. She shows an (arguable) association of some of Blake's early work with people on the sexual scene of the day, including the quack Dr James Graham, with his virility-enhancing "celestial bed", and the mesmerists, who were accused of erotic titillation in the guise of therapy. It adds up to a fantastic miscellany of sex and mysticism, though sometimes Schuchard seems to be working hard to make the pieces of her jigsaw fit.
Of course, the key phrases here are "suspicions", "supposition", "arguable" and a "fantastic miscellany".
EoR hasn't read the book, but the review gives the impression of possible link on possible link, though it certainly sounds entertaining, and Blake certainly believed in the free expression of human emotions and desires.
In one case Ms Schuchard links Swedenborg's visions (and there is no doubt Swedenborg was a direct influence on Blake's pantheon) with Blake's "Milton":
Swedenborg's belief that "the great toe communicates with the genitals" is demonstrated by Blake's self-portrait "William", showing his body flung back while a flaming star descends toward his left foot.
Of course, symbolism by its very nature does not contain a single correlative idea. David V Erdman in "The Illuminated Blake" interprets this as
Spiritually interpreted the star entering Blake is like the lightning striking the son of Job while plowing. Knowing it is impossible to receive the full inspiration of Milton by the mind alone, Blake has to go and catch a falling star. The torch of bardic prophecy is transmitted in consuming fire, burning the selfhood from foot to head, with black smoke rolling over Europe. The divine imagination's entering Blake's tarsus and his falling upon the garden path configure the conversion of Saul of Tarsus to Paul, a seizure totally annihilating the self/body.
This particular plate is also mirrored later in "Milton" by an image of Blake's deceased brother, which would make one wonder further about the sexual symbolism, if any.
So, there may well be a sexual metaphor to this image (again, Blake was not averse to such images) but it may not be the primary meaning. Nonetheless, EoR feels it's time Blake's poetry came back into fashion, and this might be the book to do it. Maybe someone will make a film of it?