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 poem's final stanza has arrived. After that whole business about "mind-forg'd manacles," these are the most famous lines in the poem.
The speaker hears lots of things, but "most" of all he hears a youthful harlot at midnight—not just any old harlot (prostitute), but one who is young (sigh) and cursing a "new-born Infant's tear."
Okay, so there's a foul-mouthed prostitute and a "new-born Infant." Is it her child? Maybe, but we don't know for sure.
But it doesn't really matter. The point the speaker is making is that babies are born into a world where young women have become prostitutes (harlots), and their tears (babies cry a lot) get cursed at instead of soothed. ("Blasts" here means something like "attacked" or "assaulted," but in a very metaphorical way. It's like saying, "I went out in the street and my ears were blasted by that guy next door's loud lawnmower.")
In addition to this whole business about children being born into a corrupt, dirty world—cursing, harlots, blasting—there's something else going on. This same harlot-curse, which "blasts" the baby's tear, also "blights with plagues the Marriage hearse."
Again, like that stuff earlier with the blood and the sighs, this is some really gnarly metaphorical stuff.
The words "blight" and "plague" are similar. They both refer to disease—a plague is, well, a plague, whereas the noun "blight" describes a kind of barrenness or infertility usually brought on by drought or disease.
But "blight" here is a verb, so we'll take it to mean something like "tarnishes," even "mars" or "destroys."
Basically, then, the harlot's curse, which is probably a symbol for her terrible life experiences (much like the soldier's sigh is for his), totally ruins the "marriage Hearse." The curse—the fact that there even is a youthful harlot in existence—completely destroys the institution of marriage. It "plagues" it, so to speak.
This is why the speaker uses the semi-oxymoronic phrase "marriage Hearse." We associate marriage with children, life, union.
A hearse, obviously, symbolizes death. Marriage is a "hearse" because, well, unmarried harlots are running around, babies seem to have no mothers (who is the mother of this baby again?), and there are no fathers to be found.
Marriage has been plagued, we might say, both figuratively and perhaps even literally. How? Well, "plague" may possibly be a reference to venereal disease, which definitely existed in Blake's day. The marriage hearse may be blighted, potentially, by the transmission of whatever diseases the harlot's profession has given her.
The harlot, in other words, engages in prostitution, which gives her some kind of sexual "plague," which she brings to her marriage (as well as the marriages of her clients).
But hey, marriage is already a "hearse" anyway (an institution of death)—at least according to this speaker—so this just adds insult to injury.
So, in the speaker's "London," life is not a bowl of cuddly babies and happily-ever-afters. Instead, it's disease, suffering, and misery. Bad times.