Very (extremely) biblical oriented, Blake must have been aware of a call, like Abraham, Moses, Samuel, Isaiah (and he certainly identified closely with Isaiah 6,) Ezekiel and many others among his biblical heroes. People are generally called according to their gifts; what were Blake's gifts?
He was trained in visual arts. His father thought to give him the very best in that area, but young Blake, mindful of the financial sacrifice involved, chose a more humble preparation-- as an engraver.
His primary calling however was in verbal art. For that there's no conventional training as such, but he was gifted as a voracious, rapid and insiteful reader. In general what you choose to read determines the kind of writing you do. Blake chose the very best literature:
"Now my lot in the Heavens is this; Milton lovd me in childhood & shewd me his faceEzra came with Isaiah the Prophet, but Shakespeare in riper years gave me his handParacelsus & Behmen appeard to me. terrors appeard in the Heavens above And in Hell beneath"(Letter 11, to Flaxman (Erdman 707-08)
Blake used the word, call, often, but where do we find an account of his calling? For what it's worth, in the Preludium of The Book of Urizen (perhaps the first Prophetic Work) he wrote:
"Eternals I hear your call gladly,
Dictate swift winged words, & fear not
To unfold your dark visions of torment."
And on page 83 "Who shall I call? who shall I send (re Isaiah 6).
That was the beginning of the Bible of Hell.
(Plate 24 of The Marriage of Heaven and Hell; Erdman 33ff)
On page 205 (Jerusalem Plate 55 - bottom of the page):
"And the Great Voice of Eternity rolled above terrible in clouds
Saying Who will go forth for us! & Who shall we send before our face? (shades of Isaiah 6 again).
Continuing at the top of 206:
PLATE 56"Then Los heaved his thund'ring Bellows on the Valley of Middlesex
And thus he chaunted his Song" (of course Blake's poetry was Los' song).
In Blake's life, like yours and mine, there's the call of God and the other call: of Satan; for Blake the most significant call of the world was what he referred to as the Main Chance: write, draw, paint in accordance with the public taste. (the Bible referred to such people as false prophets:
II Timothy 4:3: "For the time will come when they will not endure sound doctrine; but after their own lusts shall they heap to themselves teachers, having itching ears;"
If he had written all his poetry in the same vein as the Songs of Innocence, he would have become rich and famous. Likewise if he had painted like Sir Joshua Reynolds (See Annotations to the Works of Sir Joshua Reynolds; Erdman 635ff). But he was too much of his own person; he chose the Visions of Heaven rather than the teaching of Joshua Reynolds. And all true Blakeans rejoice at that final decision that he made.