Blake never tires of criticizing the Industrial Revolution (a general term to describe the rise of industry, factories, technology, and all sorts of other things), which took place over the course of the eighteenth and nineteenth centuries. Seriously. He really didn't like it. But hey, he was a Romantic poet. And those dudes were all about nature. So can you blame him?
Chimney sweeping was probably the most horrific to Blake, because it involved harming innocent kiddos who should be playing in the sun. And chimney sweeping was all thanks to the good ol' Industrial Revolution, because during that time, more people started living in cities, and as a result there were more chimneys to clean and more tiny tots needed to clean them.
Blake's criticism of the chimney-sweeper's life in this poem, and in its companion in Songs of Experience, is part and parcel of his critique of industrialism, but this one's definitely not his only poem to do so. Just check out his poem "London" to see what we mean.