What did Blake want the reader to know about the Industrial Revolution in "London"?

William Blake wrote the poem "London" in the midst of Great Britain's Industrial Revolution in the late 18th century. This transformed Great Britain into a worldwide leader of commerce. It created a prosperous middle class of manufacturers and steady wages for numerous factory workers. However, along with the prosperity came many serious problems. For instance, child labor was ubiquitous and safety standards in factories were horrendous. Although at the time the city of London, as an industrial capital, was being touted as a model of progress, in the poem "London" Blake points out the negative aspects of the Industrial Revolution by highlighting examples of people left in poverty and destitution despite the so-called progress.

Blake speaks of wandering the streets of London and seeing "weakness" and "woe" on every face he passes. He says that even infants are in fear, and that manacles, which are imprisoning chains, shackle people's minds. This can refer to the despair that factory workers feel of ever rising from their sordid state of existence. Chimney-sweepers, who were usually children, cry because of poverty and disease, young women are forced to become harlots, and soldiers shed their blood so that the rich can live in palaces.

To sum up, Blake wants his readers to know that the Industrial Revolution has its dark underside, and the people that are negatively affected by it can be found all over the streets of London.

Blake is deeply hostile to the Industrial Revolution. He sees it as indicative of an economic system that dehumanizes and degrades people, turning them into nothing more than objects of exploitation. As he wanders through the streets of London, the speaker sees victims of the Industrial Revolution all around him, their faces marked by weakness and woe. The misery of poverty is etched upon the emaciated faces of the urban poor, such as the children forced to work as chimney-sweeps for a pittance.
The desperation of London's poor is summed-up by the "youthful Harlot curse." A harlot is an old-fashioned word for a prostitute, and the young woman's loud lament could be interpreted as a withering criticism on the condition of the poor, reduced to a kind of prostitution in that they're forced to perform dangerous, degrading work simply in order to put food on the table.
https://www.poetryfoundation.org/poems/43673/london-56d222777e969