The Three Periods

In accordance with what has been said it seems appropriate
to divide William Blake's artistic and poetic career into three
periods:

1. Before age 29 he was blessed with the Heavenly
Visions.

2. For the next 20 years the visionary powers seem to have
deserted him.

3. At that point a new birth restored Blake's immagination and
faith.

(See The Vision).

The timeline is valuable here; it helps us understand what was written when:

Born 1757
1783 (at 25) published Poetical Sketches.
(This seems to be his only published work before the dismal 20 years that he mentioned in letter 51 in 1804. This letter is well worth reading several times (and with care). It casts light on where Blake was and where he was going.

So ended Period One.

Everything until the writing of Milton Letter 51 leads us to believe that Blake was working with what we might call carnal inspiration. He lambasted the bad people in every shape and form, sometimes descending to crudity. It was all brilliant, but in Blake's opinion not divinely inspired; instead he shared his values: political, social, spiritual, you name it. During Period Two, his dismal period in retrospect, following the way of the world (pursuing the main chance), he produced all of the following:

1788 All Religions are One and There is No Natural Religion

1789 Published Songs of Innocence

1789 Published The Book of Thel (1789-1790)

1790 Published The Marriage of Heaven and Hell

1793 Published Visions of the Daughters of Albion

1793 Published America a Prophecy

1794 Published Songs of Innocence and of Experience

1794 Published Europe: a Prophecy

1794 Published The [First] Book of Urizen

1795 Published The Song of Los, the Book of Los, the Book of Ahania

The fact that Milton and Jerusalem came out in 1804 and thereafter indicates that he perceived them to be different in kind from what went before; he called it taking dictation .

1804 Published Milton a Poem (c. 1804-18)

1804 Published Jerusalem (c. 1804-20)

1820 Published For the Sexes: The Gates of Paradise

1822 Published On Homers Poetry [and] On Virgil

1822 Published The Ghost of Abel

1823 Published Illustrations to the Book of Job

1827 Passed away on August 12, 1827.

Blake seemed to be telling us that none of the poetry of his twenties was dictated like Milton and Jerusalem were. The visionary power returned, and he left employment; Felpham had been essentially commercial art. He forsook it and lived in penury like so many of the greatest have done. And he proceeded to produce his greatest works.

He's said to have died with a song on his lips.