In the "Introduction" toSongs of Innocence and Experience, the model of innocence, the child, asks to hear a song (poem) about a Lamb. The child and the lamb are two recurring images/models of innocence in Blake's work.
In "The Little Black Boy," the speaker is a young black boy who knows the color of his skin has nothing to do with the content of his character. But recognizing the experience of his mother who knows the reality of racism in the world, he is given hope that he will be treated as an equal in heaven:
Thus did my mother say, and kissed me;
And thus I say to little English boy:
When I from black and he from white cloud free,
And round the tent of God like lambs we joy, (21-24)
In "The Tyger," the companion poem to "The Lamb," the speaker asks how a God could make something so fearful while also making something so innocent as the Lamb. The Lamb uses the literal image of the docile lamb but refers to the innocence of a child and also references the newborn Jesus Christ as the Lamb.
When the stars threw down their spears
And water'd heaven with their tears,
Did he smile his work to see?
Did he who made the Lamb make thee? (16-20)
In these poems, innocence and experience are two sides of the human soul. This idea of "contraries" of the psyche is evident in works like his poem "The Marriage of Heaven and Hell." A child is innocent, but when he/she becomes experienced in the world, he/she learns of things like tigers (and metaphoric predators), disease, death, grief, etc.
Other images of innocence are carefree. In "The Fly," the speaker compares his carefree life with the fly's.
For I dance
And drink & sing,
Till some blind hand
Shall brush my wing. (9-12)
The speaker, like the fly, is innocent and carefree until some experience teaches him that there are dangers and, often thoughtless, evils in the world.
Birds are symbols of innocence in Blake's work and in other Romantic poets such as Keats, Wordsworth, and Shelley. In "The Echoing Green," Blake combines images of the sunrise, the spring, and the singing of birds with youth and children playing. As children experience the world and develop into adults, they must supplement their innocent, carefree view of the world with the harsh (and sometimes wonderful) facts of earthly experience. But Blake does make the point that pure, absolute innocence exists in the carefree attitudes represented by youth, the lamb, and children.