Arguably, Rock and Roll circa 1960. Elvis had turned to Frank Sinatra-style pop after his Army hitch, Eddie Cochran and Buddy Holly were dead, Carl Perkins had never really recovered from his car accident (and was playing mostly country), Chuck Berry and Jerry Lee Lewis were reeling from sex scandals, and Little Richard had quit to become a minister. It got better.
Explicitly invoked in "The Devil Came Back to Georgia"
Johnny did you ever know that time keeps marching on
The coldest hour is the one comes just before the dawn
The Devil's Back in Georgia will you stand up to the test
Oh will you let the Devil be the best.
In choral settings of the Christian Mass, the music often lands in a minor key and slows down in the section of the Credo when Christ is crucified. When, in the next sentence, he is resurrected, the music perks right up into a major key and the tempo moves briskly along.
Without question the absolute darkest hour in Ludo's rock opera The Broken Bride is when the Time Traveller is at the apocalypse and decides to destroy his Time Machine, marooning himself there and severing himself from his beloved wife forever, to release the only things that may have a chance at destroying the zombie-making, soul-devouring Dragon: Pterodactyls.
Shinedown references this in their song, "The Sound of Madness." Potentially subverted, though, as the following line gives a potential Hope Spot:
The darkest hour never comes in the night; When ya gonna wake up and fight?!
The Manual (the Timelords' guide to creating a #1 hit single, no money or talent necessary) tells its user to schedule a week in the recording studio actually creating the single, and warns that Tuesday will be wrecked by depression and self-doubt. The only solution is to press on: by Wednesday, everything should start to make sense again.
The point of Wednesday 13's song, "We All Die:"
No matter what, you always put a smile upon my face
Just know that I love more than anything, just in case
We all die...
The movie version of the Transformers theme describes this trope quite well and name drops it:
Something evil's watching over you
Comin' from the sky above
And there's nothing you can do
Prepare to strike
There'll be no place to run
When your caught within the grip
Of the evil Unicron
Strong enough to break the bravest heart
So we have to pull together
We can't stay worlds apart
To stand divided we will surely fall
Until our darkest hour
When the light will save us all
Imagine Dragons has quite a few songs with this kind of vibe, such as "Demons," "Lost Cause," and "Nothing Left to Say."
The phone call at the end of "Young Lust" from The Wall: Pink finally gets through to his wife, only to learn that another man is answering the phone. What's more, he has to learn this through the dispatcher. At that point, he well and truly disconnects from the world around him.
"Run Like Hell" and "Waiting For The Worms" act as another one, with Pink in full-bore fascist mode, inciting his fans to violence. It is only when Pink realizes just what the hell he is doing in "Stop" that things take a turn for the better.
REO Speedwagon's "One Lonely Night": "They say it's darkest just before the dawn."