The phrase "it becomes dark" is correct and usable in written English.
You can use it to describe a transition from light to darkness, often in a literal or metaphorical sense.
Example: "As the sun set behind the mountains, it becomes dark, and the stars began to twinkle in the night sky."
Alternatives: "it turns dark" or "it gets dark."
As the lights come on in Manhattan and it becomes dark enough to watch television, he touches a button on a remote control, and suddenly the wall across from the sofa becomes a television screen.
In an ideal world, by which I mean an un-ideal world without recessed lighting and iPads, the sun sets, it becomes dark, and, presto, your pineal gland starts to release the sleep-inducing hormone melatonin (and a few other hormones).
They are joined by streams from other factories, flowing together before splitting again into smaller streams down the narrow lanes between the tenements, until these, too, eventually divide and divide once more, and it becomes dark, and the workers are home.
Each street light currently has an individual sensor which turns the light on automatically when it becomes dark.
Film will start when it becomes dark enough.
The fur is fine and light colored until PND 8/9, after which it becomes dark and coarse.
After standing it becomes darker and insoluble in water, which gives it a permanent quality.
As it becomes darker outside, images of the objects in the room behind the person are reflected dimly in the window glass.
The atmosphere changes, it becomes darker and distressing, and as a reader, it's like constantly walking on eggshells whilst waiting for the ball to drop.
But down the back alleys and twisting streets, it becomes darker, more furtive and provisional, with an out-of-season impermanence.
There's barely any room to breathe here, and the album, which lacks some of the slippery intuitiveness of Mr. Greene's earlier work, gets better as it becomes darker.
