In addition to the excellent answer above, one of the major ideas Blake presents in his work is his refusal to perceive the world in dichotomies, or parts. Good is not all good and evil is not all evil. In the poems "The Lamb" and "The Tyger," for instance, Blake presents two sides of the same God. The same God that created the lamb created the tiger. Evil is not an entity in itself, it is just another side of good.
Blake suggests in his works that people should embrace both sides of themselves, only then can we be complete.
Blake is one of the six great Romantic poets: Shelley, Byron, Wordsworth, Coleridge, and Keats. He was the first among them chronologically (sometimes he is classified as "Pre-Romantic), and his verse is most unlike the others. What distinguishes him is his Christian elements. Shelley, for example, was an avowed atheist, so Blake seems conservative by comparison.
One of his similarities to the other deals with his view of nature. I've heard him called a pantheist. Here's the Romantic view of nature ("nature of all eternity"): Peaceful, restorative qualities [an escape from industrialization and the dehumanization it creates]; Awesome, powerful, horrifying aspects of nature; Indifferent to the fate of humans; Overwhelming power of nature.
Another is the plight of the common man. Blake cared deeply for the downtrodden, the disenfranchised, the proletariate. Read his two versions of "The Chimney Sweeper" for good examples.
Also, is his use of mysticism. So says Enotes:
Considered mad by contemporaries for his idiosyncratic views, Blake is held in high regard by later critics for his expressiveness and creativity, and for the philosophical and mystical undercurrents within his work.
...some critics have speculated that Blake obscured his ideas behind the veil of mysticism to circumvent government censure.