The phrase to blot the landscape means:
– [literally] of an ugly feature: to spoil the appearance of a place;
– [figuratively] of anything unsightly or unappealing: to spoil an otherwise pleasant scene.

The phrase a blot on the landscape denotes:
– [literally] an ugly feature that spoils the appearance of a place;
– [figuratively] anything unsightly or unappealing that spoils an otherwise pleasant scene.

—Cf. also the phrase to blot one’s copybook and the adjective blotto.

António Guterres (born 1949), Secretary-General of the United Nations Organisation, was quoted as using the phrase a blot on the landscape figuratively in an Associated-Press story published in several U.S. newspapers on Friday 7^th July 2023—for example in The Boston Globe (Boston, Massachusetts):

“Investing in new fossil fuels infrastructure is moral and economic madness,” Guterres said in April. “Such investments will soon be stranded assets—a blot on the landscape and a blight on investment portfolios.”

The British satirical novelist Tom Sharpe (1928-2013) punned on the phrase a blot on the landscape in the title of one of his books, Blott on the Landscape (London: Martin Secker & Warburg Ltd, 1975), which satirised the building of motorways as a mark of social progress and featured a character called Blott.

The earliest occurrences that I have found of the phrases to blot the landscape and a blot on the landscape are as follows, in chronological order:

1-: From the following letter, by ‘an Upland Farmer’, published in the Chester Chronicle (Chester, Cheshire, England) of Friday 16^th July 1813:

ON AGRICULTURE.

Inclosing Wastes.—There is a passage in the report of the Committee appointed to examine into the state of the Corn Trade, which must strike every reflecting man in the strongest, and we trust, the most efficient manner. The Committee conclude this part of the subject, by observing, “that by the cultivation of wastes, the conversion of a greater portion of grass lands into tillage, and the adoption of a more unproved system of agriculture, the United Kingdom might be relieved from a dependence on foreign countries for corn.” […] If by a well timed application of our capital and industry, our supply was rendered (which it might easily be done) equal to our consumption, we should obtain a sinew of strength which no contingency could relax; we should at once deprive our enemies of a powerful means of annoyance, and stand upon the high ground of our exemption from any hardship in their power to inflict. Besides, by the employment of our people, who would no longer be obliged to expatriate themselves for employment, but would find ample occupation in their native country—the whole face of the kingdom would assume a new appearance—barrenness would be exchanged for fertility, and the dreary wastes which now blot the landscape, would be converted into productive corn fields and pastures.

2-: From An Essay on the Theory of Money and Principles of Commerce (London: T. Cadell, 1822), by the British political economist John Wheatley (1772-1830)—the author has just recommended that the Emperor of Russia abolish slavery and convert the slave-rent into land-rent:

By this reformation, which interest and humanity alike combine to recommend, the Emperor might establish in his own dominions, the same state of things which he so much admired in England; the same class of country gentlemen, the same gradation of orders, and the same general appearance of wealth, ease and comfort, which made him so emphatically ask “Where were our poor?” Nothing more is necessary for this conversion, than the general substitution of the law of primogeniture for the Roman law of inheritance, and the establishment of large estates, large farms, and freedom. The whole of his empire may be made to exhibit an extended succession of parks and pleasure grounds, with the accompaniment of large farms, and not only not a slave, but not a pauper be seen to blot the landscape.

3-: From The Heliotrope; Or, Pilgrim in Pursuit of Health (London: Longman, Rees, Orme, Brown, Green, and Longman, 1833), by the Scottish poet William Beattie (1793-1875):

But hark! the streets are hushed! while to and fro
Men pass in silence: gathering on the Mole
Like statues, mutely grouped, they watch the glow
Reflected from the wave; or muffled stroll
Along Chiaja—while denouncing woe
Vesuvius vomits flame—and thunders roll!
Can days so beautiful prelude such night,—
Such darkness blot the landscape of delight?

4-: From the following letter, by ‘a Benighted Traveller’, published in The Glamorgan, Monmouth, and Brecon Gazette, and Merthyr Guardian (Merthyr Tydfil, Glamorgan, Wales) of Saturday 16^th March 1833:

Sir,—It fell to the lot of that bright luminary, the Cambrian, a few weeks ago, to announce to the public, the proud and important event of Neath being lit with gas. It might with justice be asked, why it had remained in darkness so long? and with equal truth and promptness be answered, that Neath being built as it were in a coal pit, glued to its foundation with bitumen, hundreds of veins of coal streaming like meteors around, above, and below it; iron, copper, brass, zinc, tin, and other manufactures studding its environs or blotting the landscape, afforded all the requisites for immediately supplying it with gas.

5-: From The Rambler in North America, MDCCCXXXII—MDCCCXXXIII (New York: Harper & Brothers, 1835), by the British colonial administrator Charles Joseph Latrobe (1801-1875):

At the commencement of the present century, Niagara, difficult of access, and rarely visited, was still the cataract of the wilderness. […]—How is it now? The forest has everywhere yielded to the axe. Hotels with their snug shrubberies, out-houses, gardens, and paltry establishment stare you in the face: museums, mills, staircases, tolls, and grog-shops, all the petty trickery of Matlock-Baths or Ambleside, greet the eye of the traveller. Bridges are thrown from island to island; and Goat Island is reached without adventure. […]
In short, Niagara is now as hacknied as Stockgill-Force or Rydal-water, and, all things considered, the observation which an unimaginative ‘Eastern man’ is said to have made, addressing a young lady tourist, who was gazing breathlessly for the first time at the scene, was not so far out of keeping with it: ‘Isn’t it nice, Miss? Yes, all is nice, very nice, that that active little biped man has done or is doing.’
But do not imagine that we grew peevish at the sight of the blots upon the landscape, to which I have alluded, and departed in wrath and disgust.

6-: From the transcript of a statement relative to the intended improvements in the Loughs Foyle and Swilly, made by T. I. Dimsdale during a public meeting that was held in the Court house, on Tuesday 31^st January 1837—transcript published in The Londonderry Sentinel, and North-West Advertiser (Derry, County Derry, Ireland) of Saturday 4^th February 1837:

How delightful […] was the anticipation which, without an extraordinary stretch of fancy, they might picture to themselves, of those barren and unseemly slobs which are now a blot on the landscape, and generating malaria, being converted into verdant pastures and waving corn fields.