For Blake as well as in the Bible Evil is sometimes called the Serpent and sometimes the Devil and sometimes Satan.
In the Old Testament 'devils' appears three times in the Pentateuch and once in Psalms.
In the N.T. the outstanding use of the Devil came in Matthew:
-  Then was Jesus led up of the Spirit into the wilderness to be tempted of the devil.
-  Then the devil taketh him up into the holy city, and setteth him on a pinnacle of the temple,
-  Again, the devil taketh him up into an exceeding high mountain, and sheweth him all the kingdoms of the world, and the glory of them;
-  Then the devil leaveth him, and, behold, angels came and ministered unto him.
-  And his fame went throughout all Syria: and they brought unto him all sick people that were taken with divers diseases and torments, and those which were possessed with devils, and those which were lunatick, and those that had the palsy; and he healed them.
Notice that we read about the Devil in the Temptation of Jesus, but devils regarding the healings.
How Blake used Devil, etc.
PLATE 4 of The Marriage of Heaven and Hell:
The voice of the Devil
All Bibles or sacred codes. have been the causes of the following Errors. 1. That Man has two real existing principles Viz: a Body & a Soul. 2. That Energy. calld Evil. is alone from the Body. & that Reason. calld Good. is alone from the Soul. 3. That God will torment Man in Eternity for following his Energies. But the following Contraries to these are True 1 Man has no Body distinct from his Soul for that calld Body is a portion of Soul discernd by the five Senses. the chief inlets of Soul in this age 2. Energy is the only life and is from the Body and Reason is the bound or outward circumference of Energy. 3 Energy is Eternal Delight
He had more to say about it in Plate 6 of MHH.
After MHH the Devil represented more conventional meanings.
The epitome of Evil in Blake occurs in the"Great Red Dragon
with Seven heads & ten Horns [who]
of Accusations lying on the rock open before him>"
(Blake's Last Judgment; Erdman 558)
The Devil is the Mind of the Natural Frame Annotations Bacon75; E625 This work (Erdman 620-32) is well worth reading.Blake cordially despised Bacon's thought re the natural and mechanical 'frame' of the universe.
[Epilogue to Gates of Paradise] To The Accuser Who is The God of This World Truly My Satan thou art but a Dunce And dost not know the Garment from the Man Every Harlot was a Virgin once Nor canst thou ever change Kate into Nan Tho thou art Worshipd by the Names Divine Of Jesus & Jehovah thou art still The Son of Morn in weary Nights decline The lost Travellers Dream under the Hill (Erdman 269)
Jerusalem PLATE 77:
To the Christians Devils are False Religions