To begin with, it can be helpful to distinguish between "aesthetic" worldly poets/musicians and "vatic"/prophetic artists. Keats and Shakespeare, Ellington and Bill Evans belong in the first category; Shelley and Milton (and, of course, Dante) along with John Coltrane and Sun Ra belong in the second.
Blake is the foremost representative of the latter group--the bards (Milton was his hero; America's Ezra Pound his foremost descendant). Of all the so-called "Romantic" poets, he is in many respects the most atypical. Time, its passing, its presence as "personal memory," specific referents to particular places, the fleshing-out of human figures, whether upper or lower class--all this is of little interest to the visionary prophet written off as "crazy" during his life-time, eventually canonized by the Beatniks in the 1950s, and finally admitted to respectable academia. Earthly phenomena are of little interest to him because, frankly, they have no status in reality. I deliberately steer students away from his graphic art, because its symbolic nature is poorly understood by a generation brought up on images that glorify the material world (if the emphasis isn't on the "real," it's on the surreal or "hyper-real"--but the real with which today's readers identify is anything but the spiritual cosmos that Blake finds everywhere, whether a tiger or a grain of sand. (Pity his wife, who understandably had little patience with him.) More often than not, Blake's pictures nowadays detract from, rather than support, the poetry. When Blake said, "the eye can see more than the heart can know," he envisioned a human potential which few are able to realize--the sort of epiphany granted to the prophet who, after a lifetime of struggle, sees the New Jerusalem or, like Dante, the Godhead itself (the spinning wheel at the end of The Paradiso).
Blake's poetry, in both the Songs of Innocence and Songs of Experience, is music that, even when tranquil and serene on the surface, is never resolved in its minor modalities and dissonant counter-themes. In the second set of poems, that verbal music rises to a deafening fortissimo in the poem "London," in which the speaker, above all, "hears" in every cry--from the infant's to the prostitute's to the numerous thralls of the church, state and crown--a threnody of pain and suffering that climaxes in an uproar of righteous anger and indignation at the horrible realization of the consequences of "mind-forged manacles" upon the world and its inhabitants. But even in the poems from the early collection, the tone is characterized by ever-present irony--the disjuncture between the voice of the innocent child and that of the poet who knows all too well what is in store, or the disparity between the trusting faith of the child and the selfish scenarios of the "wise" keepers--the grey-haired beadles--who will violate that trust with their well-laid plans. Blake's message is unceasingly twofold--first, a testament to the holy birthright of the human child and, second, withering criticism of the "rational engines" of society that will act to estrange the child from the Father, from the Son, from its own spiritual identity.
Each of the poems may be read simply, but make no mistake about it: each is ironic and complex, inexhaustibly so. The reader must, with each passing word, be attuned, above all, to irony, ambiguity, and radical shifts in tone--or risk inflicting upon the poet the same distortions the poet finds in human society. The "enemy" is not the "Tiger" which, like the Lamb, is merely evidence of Divine Mystery and Power--but of another order. For Blake, the Lamb, the Tiger, the babe--and a poisonous reptile or virus--are created by God and are equally holy. And now the true antagonist makes its appearance: human reason and its institutions--climaxing in the state-sanctioned marriage of children and parents to the "bridegroom" of organized government and religion.
It can be discouraging to read these poems with students and discover, practically without fail, that a large majority will misinterpret them, frequently coming to conclusions opposite to the evidence of the poems individually as well as collectively. The reasons are at least three-fold: fast and careless readings of short poems that often require (and deserve) the amount of time devoted to a novel; imposition of one's own belief system (or instilled principles and conventional aphorisms); the sheer challenge offered by Blake's "radical" ideas and their deceptive expression.
Those who are serious about poetry and Blake will no doubt soon infer his "message": we must see not with the eye of reason, which measures and "charters" the flowing Thames as readily as it maps out the dehumanizing streets of London, but with the imagination, with the symbolic faculty that enables us to see the underlying spiritual basis of all material reality and thus to empathize with all living things and to live in harmony only with what is alive and vital. Blake is the first thinker I'm familiar with who puts the child first and foremost--and not until the early 19th century. For the Age of Enlightenment (The Age of Reason), children simply don't count. They have no individualism, no identity, no status in art and literature. In his own time, children were little more than the utilitarian objects of the Church-State, deployed to sweep chimneys, then disposed of. The dying chimney sweep of the first "Chimney Sweeper" poem (how regrettable that many readers do not even understand that little Tommy Dacer's "awakening" at the end of the poem is possible only because of his "murder" by the church) is, in the 2nd poem of the same title, a dead child, whose excoriating criticism includes his parents but is leveled primarily at the church. Some readers dismiss the second poem because it doesn't make sense to have a dead child lying in the snow and speaking--it's not rational.
But that's to place ourselves at the mercy of the poems' judgments--as misguided tools of Reason, deaf to the harmonious world and the discordant society around us. If it helps to postpone taking on some of the more difficult poems in either collection, fine. But each poem, each ironic line and musical phrase, each word and note of sorrow or joy is integral with the whole, each part absolutely and completely consistent with the overall theme, meaning and purpose. Seeing with the imagination requires practice and patience: reclaiming one's inner child (Wordsworth's "child-philosopher" who, trailing clouds of glory, is borne of another realm and place) is not a piece of cake. Neither is reading Blake's Songs of Innocence and Experience.