Urban legends are rumors reinforced by people's beliefs or sometimes the public fear for them, depending on what the urban legend is.
Just like that, there is an urban legend for the dark lily in the country of 'Lily', it is the name of the country which originates from their love for the lily flower. In this specific country, people believe that lilies are a symbol of peace, love, and prosperity on the other hand, they think that a lily of dark color is a symbol of conflict between the public, hatred between all and that dark lily represents that a person will fail in life if he sees one. The people in this country despise dark lilies and hate them but in reality, they don't even exist it's just an urban legend, now what will happen if the whole country fears them, will think about them and even try to avoid something that doesn't even exist? Yes, their beliefs are enough to give birth to a real dark lily.