Background (see https://greenlantern.fandom.com/wiki/Emotional_Spectrum#Blue.2FHope ):
Blue is a color close to the center, and represents the emotion of hope.
The blue light is capable of being wielded by a being who can bring
hope for the future to others. The blue light is the most powerful of
the spectrum, but it is also the most difficult to wield and understand.
The exiled Guardians
Ganthet and
Sayd were the first to harness the blue light, forming the
Blue Lantern Corps, a force which would assist the
Green Lantern Corps
in its coming trials during the War of Light. Despite being the most
powerful color, it may only function to its fullest effect in the
proximity of the Green light (hope on its own is hollow, requiring will
to make it reality). The blue light is capable of draining the yellow
light, suppressing the orange and red, and empowering the green.