It's important to know the devil, especially your own particular devil. We tend to project it; it lives in someone else (As Huey Long said, "don't tax you, don't tax me; tax that man behind the tree.") Blake certainly projected the devil in his MHH, a parody of the conventions.
For Blake the conventions are satanic; and that has a grain of truth: our preoccupation with advertising for example; our speed, etc. In particular our primitive tribalistic desire to do the same thing that everybody else does (and at the same time!).
In his early years Blake tried to exorcise these kinds of devils: Bacon's worship of Reason; Newton's belief that everything real could be weighed and measured; Locke's tabula raza; Sir Joshua Reynold's 'Nature Art'; with the greedy Industrialists who paid children 15 cents in return for ten hours of hard work every day; the slave traders who had infected the American Colonies with slavery etc, etc.
But his primary devil may have been the necessity to do banal work (for such as Hayley) when his soul longed to share with us his Divine Creations.
These things Blake projected until that moment at Felpham; I've thought long and hard about that Moment at Felpham when he realized that his devil could and would be annihilated.
He had given up his own projections and seen his own culpability; he came to himself and got his license-- from Jesus: to CREATE.