If something is a blot on a person's or thing's reputation, it spoils their reputation.

A blot is a drop of liquid that has fallen on to a surface and has dried.

If you blot a surface, you remove liquid from it by pressing a piece of soft paper or cloth onto it.

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.