Years ago, sitting in a young adult group in a Methodist church I listened to the pastor trying to convince us that Satan was a real person; a young rebel maintained that people had created Satan just like they created God. In regard to Satan the 'young rebel' was right.
Satan is the God of this World; both will pass away, but we may (will!) be translated to the Eternal Realm from which we came; such is Blake's theology.
To all intents and purposes John Milton's epic, Paradise Lost has become a part of the Bible; most Bible students have their concept of Satan and his doings from Milton. Blake was deeply influenced by the prophet he considered his predecessor; he produced a series of pictures illustrating the work:
Are these apples (or some
other fruit)? Adam and Eve are in the lower plane, Satan up in the air.
We see Adam and Eve comfortably ensconced in the lower level.
Satan seems equally comfortable on a cloud with his pet snake.
In the story in Genesis 3 Satan is not named, but from that time the Serpent and Satan are two have been seen as two names for the same entity.
One of Blake's last works, Illustrations to the Book of Job, has several representations of Satan; here's one:
Satan has just been given permission by God (Job 2 to work his will on Job, but he can't take his life.)
(You may find here in this blog the whole series of pictures of Blake's Illustrations to the Book of Job. They are scattered through the posts of this blog between June 8 and July 25, 2010.)
When writing of a personal Satan Blake generally used the term, Spectre. In general Satan is the ruler of the (material) world; the Spectre is the ruler of you and me whenever we give in to a temptation to be less (spiritually) than we can be.
Once again in the system Blake created Satan and the Spectre are to be annihilated, but people go on to Eden (or perhaps a boot camp for it).