Darkness Visible has rather a lot of shout outs and references, most of which are literary or musical allusions dating back to before 1895, when it is set. Many of these count as Genius Bonuses and are included to ground the work in the Victorian era or to reinforce the fact that the protagonist is a Gentleman and a Scholar.
Some of them, however, are purely for the reader and refer to things which, in universe, are still in the future.
The title itself is from John Milton's Paradise Lost, when Hell is first described. Marsh actually gives the appropriate quote in the book:
Marsh: "A Dungeon horrible, on all sides round as one great Furnace flam'd, yet from those flames no light, but rather darkness visible..."
Victor Hugo's Les Misérables is also quoted directly, because Lewis says it fits his thinking on being a Warden:
Lewis: ‘If the soul is left in darkness, sins will be committed. The guilty one is not he who commits the sin, but the one who causes the darkness'
Much to the surprise of the protagonist, a Bible quotation is used by the Nietzsche WannabeBig Bad as a clue to the location of the final showdown:
Sir Michael: Ah yes, this ought to appeal to you, Henry: thou thyself art a guide of the blind, a light of them which art in darkness, an instructor of the foolish...’
I raised an eyebrow at him. ‘Bible quotations, from you? What sort of clue is that from an irreligious man?’
‘It’s from Paul’s letter to the Romans,’ Marsh said, after a few seconds’ intense thought.
The Bible is referenced obliquely several more times, such as when Lewis says that he feels as old as Methuselah.
Marsh gave a theatrical sigh. ‘Once more unto the breach, dear friends, once more!’ he proclaimed.
Or close the wall up with our English dead... I finished the old phrase silently, and did not find it to be an encouraging thought.
The first time we encounter William Marsh, Lewis describes him by referring to a relatively obscure self-portrait by Dante Gabriel Rossetti. This one, in fact.
Marsh quotes from the speech Oscar Wilde gave in court during his indecency trial:
Marsh: 'Ah, Lord Lewis, we shall never know another like him,’ Marsh cried. ‘I shall never forget the speech he gave in court. “The love that dare not speak its name”. How could such an orator be a criminal? I confess I broke into applause right there in the gallery.'
Lewis quotes Newton's Second Law of Motion to test Marsh's education, and is delighted when the younger man gets the reference.
Later, he quotes the famous phrase "a little learning is a dangerous thing" during a heated argument, only to have Marsh throw it back at him with "Hang Alexander Pope!"
Charles Darwin's On The Origin of Species is a major plot point.
As is the ballet Swan Lake, which appears twice and may be considered to mirror the book's plot in many ways. Specific details are given about the performance which the characters attend at the Mariinsky Theatre, including the name of the prima ballerina and mention of the 32 fouettes en tournant which she was the first to perform in the black swan pas de deux.
Many pieces of music are referred to, usually ones which fit the mood of whichever scene they occur in. Examples include: Corelli's La Folia, Balakirev's Islamey, Beethoven's Spring Sonata and Kreutzer, two different pieces by Purcell and the Music Hall favourite Little Annie Rooney.
One of the characters reads aloud from Spencer's Faerie Queene to entertain the rest.
Lewis reads Jules Verne's Carpathian Castle, as well as, in one of many historically accurate touches, the most recent edition of Punch magazine at the time.
The works of Charles Dickens are mentioned (this might actually be mandatory for late-Victorian works).
Lewis mentions the investigation of the Jack the Ripper murders, with an in-universe twist which suggests he was personally involved:
Lewis:I was virtually certain the Ripper was a venturer, to have escaped the scenes of his murders unseen, but our investigations at the time had turned up nothing and we had never caught him.
One of the characters compares Lewis, scathingly, to Sherlock Holmes, as a shorthand insult for not catching a criminal sooner.
Mention is made of the Second Afgan War, as well as (repeatedly) to The British Empire in India.
Classical mythology is referred to several times, including Lewis's comment that he can't predict what's going to happen because he isn't a Delphic Oracle.
The reference to a 'solitary, optimistic man selling sausages in a bun' is a shout out to Terry Pratchett's CMOT Dibbler.
Sam Vimes's comment that "We who think we are about to die will laugh at anything" is referenced by Lewis (with a little bit of fudging to get round the fact that Terry Pratchett hadn't even been born yet):
Lewis: I cannot recall who it was, but I am sure a great man once said that those who are about to die will laugh at anything, and I have certainly found it to be true.
The name Albert Spangler is also a Discworld reference.
Lewis's shout of "It's a trap!" is a shout out to Admiral Akbar and the related meme.