"The Chimney Sweeper" begins on a mysterious note. The first sighting of the young chimney sweep is a kind of grim vision, the boy made initially unrecognizable by the sheer amount of soot that covers him. Instead, he is just "a little black thing among the snow." This vague description has a kind of distancing and dehumanizing effect, showing the way that the boy has been made literally and metaphorically unidentifiable—as though he is no longer a child, but a mere black shape, a dark mark on the conscience of society.
This first line uses also delicate consonance:
A little black thing among the snow,
The gentleness of this reminds the reader that, though the child speaker is world-weary and experienced, he is still small and vulnerable.
The second line points out that the "little black thing" is crying and weeping. Consider who is the observer here; most likely, Blake intends to put the reader in that position as part of an argument that everyone bears some responsibility for the way society works (or, indeed, doesn't work). That is, the reader becomes a spectator to this small child's suffering.
The poem here also uses epizeuxis in the repetition of "weep! 'weep!" This is an echo of the other "The Chimney Sweeper" poem, which is found in Songs of Innocence (this poem comes from Songs of Experience). It's an important link because this poem essentially undoes the false moralizing of the first poem. In the first poem, the terrified young chimney sweeps also "weep" but find (temporary) comfort in the idea that God will grant them happiness if they only work obediently and dutifully.
Soon after, though, comes this poem's picture of abject misery—with the quick repetition of "weep" implying it might even be one of the boys in the first poem—with the key difference that now he is wise to the lies and deceptions of organized religion. The assonance of "notes of woe" is also an important factor here, making the line itself sound like a kind of depressing song. The alliteration of "weep" with "woe" links further the action of crying to the sweep's life of misery.