It is a good question though. Why does Rumia – a yōkai of darkness – have a spell card that is very light in theme? 🤔 Light can be darkness though, I suppose, if it blinds you. 'Cause that seems to be the theme? She also has a night blind bird thing (i.e. Mystia) there too.
So Rumia's darkness is metaphorical too. It's not just the black stuff.
