In the poem "London," the speaker says that as he walks through the city of London, he sees "in every face ... Marks of weakness, marks of woe," and hears, in "every voice," the clinking of "mind-forg'd manacles." He describes "chimney-sweepers" crying, and he says that the blood of "hapless" soldiers "buns in blood down / Palace walls." The overall impression here is of people blighted and burdened by abject poverty. In eighteenth-century England, children as young as five would be sent down chimneys as chimney-sweeps. This was an incredibly dangerous and difficult job that these children were forced to undertake because their families were so poor.
The soldiers' blood running down "palace walls" also suggests that the rich, represented by the "palace," are responsible for the deaths of the poor, represented here by the soldiers. These soldiers were sent to fight wars for the interests of the rich. The rich have the blood of the poor on their walls, as one who is guilty of somebody else's death might be said to have blood on their hands.
The abject poverty that Blake protests against in this poem, and the exploitation of the poor by the rich, are not concerns relevant only to Blake's own time. They remain relevant, unfortunately, to London today. It is estimated that approximately 2.5 million people currently live in poverty in London. In one area of London, Tower Hamlets, approximately 57% of children live in poverty.
In 2017, a fire broke out in a tower block (Grenfell Tower) which was home to some of London's poorest citizens. The fire spread because of the cladding on the outside of the building. This cladding was known to burn easily and had failed numerous fire tests, but it was installed anyway because it made the tower block look more attractive for the wealthy Londoners who lived nearby. There is perhaps a direct link here to the blood of the soldiers running down the "palace walls" in Blake's poem.