If something is a blot on a person's or thing's reputation, it spoils their reputation. a blot on the reputation of the architectural profession This drugs scandal is another blot on the Olympics
an act that brings discredit to the person who does it; "he made a huge blot on his copybook"
If you describe something such as a building as a blot on the landscape, you mean that you think it is very ugly and spoils an otherwise attractive place. The developers insist the £80m village will not leave a blot on the landscape