It's just called "London." That's simple enough, isn't it? It's amazing that one word makes us think of so many things: the Thames, Westminster Abbey, the Crown Jewels, Big Ben, Charles Dickens, bad food, rain, etc. It's reasonable to assume that, when he wrote this poem, Blake was definitely planning on his readers thinking of all, or at least some, of those things.
So the very word "London" conjures up so many things, none of which, come to think of it, are actually described in this poem. That magical word "London" is kind of like a fishing lure. It attracts our attention, dazzles us with its sheen, and then—Wham! We're snagged by something unexpected. It seems like a cruel little trick, but Blake is no different in this regard than so many other artists.
In general, the word "London" leads us to believe that this poem is a little travelogue of sorts, a description of a place. And that is essentially what it is, but it is a very narrow, or "charter'd," in the poem's words, view. We see London, sure, but it's the city's dirty underbelly, the dark parts that nobody talks about at parties. If the poem had been titled "The Gross, Filthy, Bad Things About London," would you want to read it? Meh, maybe not so much.