In William Blake's poem, "The Chimney Sweeper," the metaphor of the "coffins of black" can be seen to represent innocence. This can be justified by the fact that the speaker in the poem has been sold as a slave.
And my father sold me while yet my tongue
Could scarcely cry " 'weep! 'weep! 'weep! 'weep!"
So your chimneys I sweep & in soot I sleep.
Later in the poem, readers see the metaphor in question:
Were all of them lock'd up in coffins of black.
Here, the numerous sweepers are described as living a life confined by sweeping. The sweepers have nothing else in life but to sweep.
In the following stanza, and the one which shows how the coffins represent loss of innocence, an Angel frees the sweepers from their life of confinement and loss of innocence.
And by came an Angel who had a bright key,
And he open'd the coffins & set them all free;
Then down a green plain leaping, laughing, they run,
And wash in a river. and shine in the Sun.
It is in this stanza where the reader can see how the escape from the "coffins of black" represent a loss of innocence. It is when the sweepers are freed that they are able to leap, laugh, and run--free as all children should be.
Therefore, the coffin truly represents a loss of innocence and not innocence itself.
"Coffins of black" represent chimneys first of all. Perhaps viewing the phrase as a representation of innocence is looking too deeply.