I'm not sure where or when I read this. Probably 1980s or 1990s, but it might have been a magazine or an anthology.
The story is set on a post-technological (fallen) world where technological advancement is blocked by an unexplained invisible aura that might be called "shadow." The aura, which makes people sick, will grow on anything made, but the more artificial it is, the faster the aura grows and the stronger it becomes. If I'm recalling the back story properly, a colony starship landed, but before they could accomplish anything the entire ship was enveloped, and the colonists had to flee with essentially the clothes on their back. Generations later, there is a society existing at approximately mediaeval level; they haven't lost all knowledge, but can't use anything more advanced than simple hand-worked metal tools.
The protagonist is one of a special guild who fight these auras. Because he is specially sensitive, he sees the auras as a dark haze around objects, and can easily identify them and tell how strong they are. He travels from town to town, destroying the auras on people's tools so they can keep using them. He does this by a psychic effort, fighting the aura into submission; it is very draining and he can only do so much in a day. I remember a mention of how fighting the aura means taking some of the darkness into yourself, which affects his mental state and maybe his health.
In one town he becomes alarmed when he finds they have far too much metal for one of his guild to keep the auras under control. He is introduced to someone who claims to have a new way to fight them, which I recall as "mindlight." Instead of dominating the dark aura, this person creates a brightness that drives it away. When this is demonstrated, the protagonist is temporarily blinded; his connection to the aura has made him sensitive to the brightness too.
I'm not sure where the story went after that; I recall one more scene but it may have been from a sequel.
In this scene an enemy has managed to create a large number of swords and is threatening to conquer a town. The swords are stored in a tent in barrels so the people who drive away the auras can keep them clear, and the protagonist's side sneaks in and dumps a huge amount of powdered metal all over everything. Because of mindlight they can manufacture it, but the other side with the traditional way of dealing with auras is overwhelmed. (The resulting aura was bigger than the tent and visibly expanding as they snuck away.)