If you've read our "In a Nutshell" section, you know Blake wrote a lot about children. Children are everywhere in his poetry as characters, and many of the poems in Songs of Innocence and Experience resemble children's poems. "London" is no different. There are several infants mentioned, as well as a chimney sweeper (back then, that deplorable job was reserved for children). Strangely, the children in this poem aren't really doing childish things. Okay, they cry, but their cries are cries of fear, or tear that are corrupted by a harlot's curse. Instead of playing, the children work. The basic idea is that the world is upside down, and that is obvious in the fact that children aren't really children.
Line 6: The speaker hears cries of fear from London's infants. Those cries, he notes, apprise him of the presence of "mind-forg'd manacles," which is to say that they somehow symbolize or conjure up the idea of slavery and confinement. Infants and slavery? That's no good.
Lines 9-10: The less specific slavery alluded to in line 6 is made more literal here. Chimney sweepers were usually children. They don't sing as they work, but cry, and their cries "appall" the church, which is to say, cast shame on it. The church, for its part, is metaphorically black, underscoring its role in the mistreatment of children.
Lines 15-16: The infant's "tear" is blasted. "Blasted" is a metaphor for the way in which the people's innocent crying is marred or fouled by the inappropriate, gross cursing of a nearby harlot. Things are bad when a baby can't even cry in peace, the speaker implies.