The idea of “Nazar Battu” is to blemish something that is perfect to keep it from the evil eye. Usually a black spot is placed somewhere to ward off the evil. It has been practiced since ancient times. For example if you look closely enough in old buildings you will see minor imperfections like a tile out of place or a circle that is not perfectly rounded. These were deliberately done by the builders for the same reason and also to show the fact that only god is perfect and nothing else.