My trailer is a 2011 Sonnybrook Harmony 21FBS. Overhead lights are dual fixture LED type with a slide switch. There are 3 lights of this type in the center of the trailers (others in there as well, ...
Have you checked all through the trailer for a wall type switch? Even though there is a switch on the fixture, power may be controlled through a switch on a wall or on a cabinet.
If there is no wall switch and the blue wire has no power, I guess you're destined to pull 12V power from another location.