To a level 70, the dark blue/light blue cut-off is around level 54. I cannot say with certainty if it is level 54 or 55, but it is right around that. Most of the trash mobs in The Bloodfields near the Dranik Scar zone line are light blue to my level 70 characters and these mobs are listed as 53-54 in the beastiary.
To confirm this, as a level 70 character, just con people in PoK where you can /who them to see their level.