"Coffins of black" represents innocence and what is done to innocent children.
In the poem, the narrator, himself a young chimney sweep, recounts that poor Tom Dacre had a dream in which he saw thousands of chimney sweeps locked in "coffins of black." If Tom stopped there, the reader could easily think that the little boy, at least subconsciously, was wising up about his situation and that of the other chimney sweeps. They spend their days in the "dark coffins" of soot filled chimneys, which they clean by climbing through and brushing. They have to be kept half starved to fit down the narrow chimneys, and they have a high rate of cancer from their contact with so much coal dust. Their work truly is like being locked in a black coffin.
However, what makes this image heartbreaking is that it is followed in the next stanza by the rest of Tom's dream. Because he is still so innocent, he dreams an angel comes and sets the chimney sweeps free. We adults, who are experienced with the evils of...
(The entire section contains 3 answers and 602 words.)