The term 'blot on the landscape' is a metaphor for something that is an EYESORE, a STAIN, a MAR, or a STIGMA. An EYESORE is something that is unpleasant to look at and can ruin the beauty of a landscape. A STAIN is a discoloration that is difficult to remove. A MAR is a blemish or defect that detracts from the appearance of something. Lastly, a STIGMA is a mark of disgrace or discredit that is associated with a person or thing. All of these words describe something that is an unwelcome addition to a landscape.