It's pretty clear (from the Silmarillion and other texts) that the "darkness" is intended semi-metaphorically. It refers to Sauron's evil as creating a figurative darkness over all he commands. Note also that his domains are always described as dark and brooding, even during the daytime.
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
...
'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
...
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
...
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
