I'm inclined to believe from all the in-universe interviews and observations that Rumia is actually far more intelligent than she lets on. She gives the impression of being dumb as bricks by making her surroundings too dark to see causing her to bump into trees, but at the same time she also shows that she actually does have a very good control of the darkness and can really see at such dark places that not even Aya could see anything in.
It's also ironic that the man-devouring hunter youkai reputation went to the one character who has stated that she finds it a bother and boring to hunt humans.