Near where the chartered Thames does flow.
And mark in every face I meet
Marks of weakness, marks of woe.
In every cry of every man,
In every Infant's cry of fear,
In every voice: in every ban,
The mind-forged manacles I hear.
How the Chimney-sweeper's cry
Every blackning church appalls;
And the hapless Soldier's sigh
Runs in blood down Palace walls.
But most thro' midnight streets I hear
How the youthful Harlot's curse
Blasts the new-born Infant's tear,
And blights with plagues the Marriage hearse.

The desperation of London is summed up by the young prostitute cursing. Is she cursing as in swearing, or cursing others, like the men who exploit her? Or is she herself, also cursed? 
 Her curse furthermore harms the new-born infant.  “Blasts” would suggest that her own howl of anger and grief frightens her child; she doesn’t take his tears into account.  She may be cursing the child himself or her fate.  But Blake’s main point is in the final line: her curse calls down judgment on the poor state of marriage at the time (infidelity was taken for granted by many men) and turns the carriage ridden by newlyweds into a hearse. 
 The word “youthful” here is a direct criticism of the conditions for young, working-class women in the time of George III. Poor urban women worked as seamstresses (which required skill that took time to learn), and in domestic service. 
 The early industrial revolution, (the ‘Romantic’ era) when Blake lived, started in the north of England;  factory work for women and children was beginning to grow and mill towns to spring up, though conditions in the mills and factories were terrible. But in London those who failed to find work sewing or in domestic service fell back on prostitution as the most viable means of subsistence. This was the fate for thousands of young women with nowhere else to turn. Before taking up the profession, this young unencumbered girl might have been an ‘English rose’ as opposed to her now haggard, cursing self.
London