Background (see https://greenlantern.fandom.com/wiki/Emotional_Spectrum#Black.2FDeath):
Black is the color of the dead and emotionless. It is the color of the
Black Lantern Corps.
Black, technically not a color but more an absence of color and light,
does not appear on the spectrum itself for this reason, as the Emotional
Spectrum is a light based energy classification. Those dead or once
dead are capable of becoming
Black Lanterns.