If I wanted to express something adds to the beauty of a place I'd use words like: embellish, adorn, beautify, et cetera.
What is the word which is used to express that something decreases the beauty of the place?
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Tarnish has the added implication that object in question was bright, shiny, and beautiful to begin with. merriam-webster
I prefer to use sully: to damage or ruin the good quality of (something) Merriam-Webster
uglify is commonly used, at least among programmers, as the opposite of beautify. It is, itself, an ugly word, and ideally would not be your first choice in any literary context; however, all the other examples given (tarnish, blight, etc.) all have further implications - e.g. tarnish literally means to lose brightness, while blight implies the subject is a plant.
I think that mar is probably the best word, but for pure technical precision, you can't beat uglify.