Blake has many characters; to understand Blake we need to become acquainted with his characters. One good way to do that is to put his name on the Concordance; then review the occurrences of the name. This will give you at least an introductory acquaintance with the character.
The first occurrence of Orc per se is in America, a Prophecy, 1.1; E51 with:
"The shadowy daughter of Urthona stood before red Orc." If you read America you might conclude that Orc was a symbol or icon of Revolution.
"The red flames of Orc," pictured on America Plate 10.
He was that, but as Blake's corpus progresses, we find that Orc was other things as well -- many other things in fact.
Los, through his emanation, Enitharmon, gave birth to Orc, his first born. (This is not a matter of a human birth, but a figure of speech, as if
the Boston Teaparty gave birth to the American Revolution.)
But Blake carried the figure further; he described an archetypal father and son suffering under the Oedipus complex. Los, out of jealously, took Orc to the top of a mountain and chained him there to a large rock. What in the world is Blake trying to say? Your guess, but I might think he's expressing his disenchantment with Revolution.
As a youth Blake, like a lot of Brits, was a fervent (American) patriot. Likewise the French Revolution until the guillotine became common; disillusionment struck and Blake took off his red cap.He also wrote a poem expressing more pointedly his feelings about the matter:
"The hand of Vengeance found the bed
To which the Purple Tyrant fled;
The iron hand crush'd the Tyrant's head
And became a Tyrant in his stead."
If you have access to Damon's Blake Dictionary, you may find much more data on Orc than this humble post can provide.
Good luck and let me know what you think.