Synonyms for Lightless (same or very similar meaning)
WordNet sense 1 (impossible or nearly impossible to see; imperceptible by the eye):
invisible, unseeableWordNet sense 2 (without illumination):
unlighted, unlit, unilluminatedWordNet sense 3 (devoid of or deficient in light or brightness; shadowed or black):
darkFrom the ODE community, based on WordNetadd/edit