If you wanna get technical.... the blue (and red) lines are actually "inside" the regular square outline. You can just see the grey dividing line on the outside of the blue.

Not so clear on this image but check out your tiles or ibsh's map files. You can clearly see it there.

That makes it fairly clear which spot the blue line refers too.

I think any blue shading seems to be a by product of the art underneath... eg a river.