While it is possible that the poem could be about other cities, at the time of writing (1794, contained in Blake's collection Songs of Experience), London was one of the largest cities in the world, and certainly one of the largest cities in Europe. Unlike many of the poems written for the collection Songs of Experience, the poem "London" does not have a corresponding poem in Songs of Innocence, suggesting the poet could not bring himself to write about London from that perspective.
Since the poem's title names the city he writes about, it is not necessary to mention it within the poem itself. But it's plausible to suggest that even without that title identifying the city, most readers would understand that Blake is describing London. The imagery is specific to London, including the Thames (the river that flows through the city that makes up a significant portion of its landscape), and the capitalization of the word "Palace" suggesting Buckingham Palace. As well, the reference to "every blackning church appalls" suggest a major industrial city where soot would blacken stone walls, and at the time London was the center of industry in Europe.
The poem could certainly serve to reflect the horror and drudgery of living in a large city during the time period at the dawn of the Industrial Revolution, which took place between 1760 and 1840, beginning in Great Britain.