The tone of a poem is the emotion it communicates. The tone of Blake's poem is one of bleak and hopeless sadness at the distress he sees everywhere in London.
Blake conveys this sadness at the state of London through word choice. In stanza one, words such as the "weakness" and "woe" seen in "every" face tells us that London is drenched in sadness.
In stanza two, the repetition of the word "cry" applied to "every" adult and every "infant" (young child) accentuates the feeling of sadness. Nobody in the city is happy. "Mind-forged manacles" (handcuffs) imprison everyone. These are not literal handcuffs, but a mindset that accepts being made miserable.
By stanza three, the mood is turning from sadness to anger, indicated by the use of the word "appalls" and the angry image of blood running down palace walls. In the final stanza, words such as "blight," "blast," "curse," and "plagues" convey the idea that the city is diseased.
We note, too, that Blake uses alliteration to call attention to the words that convey tone. Alliteration is placing words that begin with the same consonant close together. In stanza one, for example, "woe" and "weakness" are paired alliteratively, while in stanza two, our eyes fall on "mind-forged manacles."
The poem expresses Blake's condemnation of the human suffering he blames civilization, including the church, for bringing to humans. Like most Romantic poets, Blake sides with the underdog and prefers a more natural world to the urban blight he sees all around him.
It would be possible to write a poem that praises all the grandeur, power, and beauty of a great capital city like London (and this was often done), but Blake's focus is, instead, on the weak and suffering, for whom he want the reader to feel pity and concern.