In this post we talk about that weird blue tape found on Subaru wiring harnesses. This will be a rather short one, but it's a pretty helpful piece of knowledge. The blue tape is for the manufacturer to have components that are found in all harnesses, but are not used on that vehicle. Commonly you'll find things like the rear accessory power supply, fog light relay relay (pink plug in the main picture) or connections for an alarm or cruise control (see below).
When we merge harnesses, we also include blue tape for connectors that may not be used like expansion for DCCD, emissions equipment, or immobilizer.