The comparison of Tom Dacre with a lamb symbolizes his innocence. In Blake's poem, "The Lamb," he describes the Lamb as a harmless creature, one with the "softest clothing" and a "tender voice." The speaker in "The Lamb" also notes the connection to Christ as the "Lamb of God." So, in "The Chimney Sweeper," the comparison of Tom to the Lamb symbolizes innocence and a connection to God. The "lamb" is, in these cases, an symbol of innocence and an allusion to Christ (who represents innocence and salvation).
The images of black and white, symbolizing bondage and freedom, are mentioned in the first three stanzas. Tom's white hair is shaved so that the black soot can not soil it. That night, Tom has a dream that the sweepers are locked in black coffins:
As Tom was a-sleeping he had such a sight!
That thousands of sweepers, Dick, Joe, Ned, & Jack,
Were all of them locked up in coffins of black;
The images of white and light symbolize innocence and the images of black symbolize death and dirt. (There is also a possible inference that the black soot on the children sweepers makes them appear African. At the time, slavery was legal but Blake was an adamant supporter of abolition; therefore, he might have inferred this comparison but was only using it as an expression of his opposition to slavery.) Therefore, washing the soot off symbolized a transition to freedom. Consider this freedom in the context of slavery or in the more literal context of these children being freed from servitude as chimney sweepers. The symbolism of light and white images continue to represent freedom in the next stanza. The Angel comes with a "bright" key and after washing in a river, they "shine in the Sun." The idea is that, for the children chimney sweepers, the only escape from their servitude is through dreams or death (Heaven).