The blue key is for a box in the basement of this house located here
ah now I feel dumb I didn't even know that part of the map existed. I ran into what I'm assuming is a bug when I finished the game though, where I could clip through the door in the basement that the main character doesn't remember being there... after a few minutes it went to the end screen but it made the ending a little lackluster. Otherwise I had plenty fun and was thoroughly spooked. Good game!