They will only glow if i stay in the zone during the transition from day to night. If I waypoint into an area that is already in the night cycle my ghastly weapons will not glow. Even if I stay in a zone when it switches to night and they do start to glow, teleporting long enough distances (even on the same map) for there to be a loading screen will remove the effect.
Also the effect never shows in dungeons which is disappointing.