“blind boy” = protagonist
“helps police identify and apprehend a possessed serial killer” = inciting incident
“must prove he witnessed the crimes” = main character goal

Seems the playfulness of this logline, might be the contrast of blindness vs. sight. How a blind individual’s witness of a crime would probably provide less useable evidence. But how this might link to a demonic entity, as the real homicide culprit, is the real challenge.

An important quality of story, is for the audience to first empathise with the main character. Tell us a little about the boy, not just his blindness, but one or two words describing his psychology. Courageous, weak, adventurous, timid … these are relatable qualities. That determine how he might relate to inciting incident events. That affect how he might emotionally respond to main goal struggles.