Thomas Taylor, the Grecian

According to the letter that Blake wrote to John Flaxman, Milton, Shakespeare, Boehme and 
Paracelsus were his teachers, but there were others:

Unlike the ones above Thomas Taylor was Blake's own age.  About 1784
Taylor gave 12 lectures at Flaxman's home on Neoplatonism.
Blake undoubtedly attended (at least some of) these lectures.

Blake hitherto had frowned on Plato, but he was impressed by Taylor's lectures.
He found a new world in Plotinus and the others; they exercised a marked
influence on his poetry and pictures.

On page six of Milton Percival's Circle of Destiny he quoted this passage from Plotinus:
"No: our reasoning is our own.  We ourselves think the thoughts that
occupy the understanding--for this is actually the We--but the operation
of the Intellectual Principle enters from above" (sounds remarkably like WB).

The same year (1784) Blake wrote his satiric parody: An Island in the
Moon: he referred to Taylor as Obtuse Angle and to himself as Quid:

" Obtuse Angle came in Oh I am glad you are come said quid......
Obtuse Angle giving a grin said Voltaire understood nothing of the 
Mathematics and a man must be a fool ifaith not to understand the 
Mathematics .........

"But Ob[t]use Angle, entering the room having made a

gentle bow, proceeded to empty his pockets of a vast number of
papers, turned about & sat down   wiped his [head]
 with his pocket handkerchief & shutting his eyes began to
scratch his head--well gentlemen said he what is the cause of
strife   the Cynic answerd. they are only quarreling about:
Voltaire--Yes said the Epicurean & having a bit of fun with him.
And said the Pythagorean endeavoring to incorporate their souls
with their bodies 
  Obtuse Angle giving a grin said   Voltaire understood nothing
of the Mathematics and a man must be a fool in faith not to
understand the Mathematics"  

Read it!

Blake had derogatory things to say about Voltaire:
he mentioned him 25 times; check the Blake Concordance; here are a couple:
"Mock on mock on Voltaire...." (the Notebook).
"Her Two Covering Cherubs  afterwards named Voltaire & Rousseau"

In spite of Blake's ambivalence re Greek  art he depended on it to a great degree;
look for example at the Arlington Tempera: Kathleen Raine demonstrates just how
Greek it is.  In Blake and Antiquity she showed how Blake used several Greek
myths including the Cave of the Nymphs, the myth of Psyche, the myth of 
Persephone and the myth of the Great Year.

Blake's friendship as a young man with Thomas Taylor  is largely responsible for
the enormous amount of Greek thought that fills his pages and his art.