A friend sent a clipping from the Washington Post re a lecture to a couple of thousand people in GW re Al Gore's latest book, The Assault on Reason. As the reporter perceived it he speaks a language impenetrable to large numbers of people, probably the majority: too many 5 dollar words all grouped together. So it was with William Blake.
People as well read as Gore can understand his language. To understand Blake you might best be acquainted with the Bible, Homer, Plato, Plotinus and the other neoplatonists, the Gnostics, Boehme, Paracelsus, Shakespeare, Spencer, Milton, and a few other writers. His poetry draws on all of the above.
Now you might spend years reading Blake with ever increasing pleasure, but the time might come (you might be about 80) when you feel led to look at some of these sources.
In that case your best bet would be to get a little book by Kathleen Raine called Blake and Antiquity. It's actually a condensed version of Vol II of a much larger book, part of the Bollingen Series, called Blake and Tradition (long out of print).
In particular Raine gives a study of four myths:
The Cave of the Nymphs
The Myth of Psyche
The Myth of Persephone
The Myth of the Great Year
Taken together they give a good glimpse of what Blake is really talking about. She provides many illustrations from Blake's works, pictoral and verbal. With Raines' help you can see the underlying meaning in each of them.
You might also feel led to go to Ram Horn'd with Gold for a systematic (forbidden by Blake) overview of his writing. You might even reach the point of giving the author a hand in his ongoing revision.