“A person knows when they’re in darkness, even when they can’t see. -Nightblood”

Source: The Concept of Mind (1949), Ch. VIII: Imagination, (2) Picturing and Seeing

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Source: City of Fallen Angels

Misattributed
Variant: When its dark enough you can see the stars.

Widely attributed to Ralph Waldo Emerson on the internet; however, a presumably definitive source of Emerson's works at http://www.rwe.org fails to confirm any occurrence of this phrase across his works. This phrase is found in remarks attributed to Charles A. Beard in Arthur H. Secord, "Condensed History Lesson", Readers' Digest, February 1941, p. 20; but the origin has not been determined. Possibly confused with a passage in "Illusions" in which Emerson discusses his experience in the "Star Chamber": "our lamps were taken from us by the guide, and extinguished or put aside, and, on looking upwards, I saw or seemed to see the night heaven thick with stars glimmering more or less brightly over our heads, and even what seemed a comet flaming among them. All the party were touched with astonishment and pleasure. Our musical friends sung with much feeling a pretty song, “The stars are in the quiet sky,” &c., and I sat down on the rocky floor to enjoy the serene picture. Some crystal specks in the black ceiling high overhead, reflecting the light of a half–hid lamp, yielded this magnificent effect."
Misattributed

Source: Volkhavaar (1977), Chapter 9 (p. 78)

Source: The Book Thief

"The No Point", p. 256
Sun Ra : The Immeasurable Equation (2005)
Context: When the person Myth meets the person Reality
The spirit of the impossible-strange appears
In dark disguise
It is always there where nothing inverts itself
and becomes something
Whatever is the imperative need