The blue wire goes to a signal source that is active when the bike is on, and dead when the bike is off. That could be the parking light up front which is on when the key is on, or the low beam feed which only comes on after the engine starts running.
All Joe's diagram does is show the wiring for installing a separate a fuse box that is outside the CanBus system and is powered up automatically. Any accessory lights, radios, intercoms, horns, etc. are wired to the new fuse box, but none of that is shown in the picture above.
However, it doesn't address the original question about trailer lighting. To allow for running (tail) light and brake light connections without upsetting the CanBus, you need a relay isolation package like this one
or the one mentioned on the bottom of this page
. It will use the existing bike wiring to sense when the tail lights and brake lights are on, but will power the trailer lights directly from the battery.
I also see an isolation relay listed here
, but I'm not familiar with that company.
More info can also be found in this thread
in the Trailers, Sidecars, & Trikes
Adding a separate fuse block is a good idea, but is a whole different discussion than trailer hookups.