É bem claro (do Silmarillion e de outros textos) que a "escuridão" é pretendida semi-metaforicamente. Refere-se ao mal de Sauron como criando uma escuridão figurativa sobre tudo o que ele comanda. Note também que seus domínios são sempre descritos como sombrios e chocantes, mesmo durante o dia.
Hail Eärendil, bearer of light before the Sun and Moon! Splendour of the Children of Earth, star in the darkness, jewel in the sunset, radiant in the morning!' - Silmarillion
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'Four days and nights, and on into a fifth, we rode from the Black Stone,' he said. 'And lo! in the darkness of Mordor my hope rose; for in that gloom the Shadow Host seemed to grow stronger and more terrible to look upon. - LOTR : The Return of the King
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The Men of Númenor were settled far and wide on the shores and seaward regions of the Great Lands, but for the most part they fell into evils and follies. Many became enamoured of the Darkness and the black arts; some were given over wholly to idleness and ease, and some fought among themselves, until they were conquered in their weakness by the wild men. 'It is not said that evil arts were ever practised in Gondor, or that the Nameless One was ever named in honour there - LOTR : The Two Towers
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Gandalf fell silent, gazing eastward from the porch to the far peaks of the Misty Mountains, at whose great roots the peril of the world had so long lain hidden. He sighed. There I was at fault,' he said. I was lulled by the words of Saruman the Wise; but I should have sought for the truth sooner, and our peril would now be less.' We were all at fault,' said Elrond, and but for your vigilance the Darkness, maybe, would already be upon us. But say on!' - LOTR : The Fellowship of the Ring