One picture carries this whole word: light leaving. Not light that has already gone — light in the act of going. The sky before rain, a theatre as the lamps go down, a screen out. The word is made for the middle of that change, which is why English keeps 'it got dark' for the plain fact and reaches for 'darken' when it wants you to watch the change happen. Two more things in the picture matter. First, it works in both directions with no change of shape: the sky can darken by itself, or clouds can darken the sky. Second, once you can see the picture, you can see why the same word is used for a colour going deeper, for hope going out of a room, and for a face closing over.
The source traces 'darken' to Middle English derkenen, from dark plus the ending -en. That ending makes a verb of change out of an adjective, so dark plus -en is literally 'go dark' or 'make dark'. The modern core is still exactly that, which is why the word insists on the change rather than the state.
來源記載 darken 來自中古英文 derkenen，也就是 dark（暗的）加上字尾 -en。-en 的作用是把形容詞變成「變成……／使變成……」的動詞。所以這個字從一開始就不是在講「暗」這個狀態，而是在講「往暗的方向去」這個過程。今天的核心畫面正好還是這樣。
Wiktionary entry for darken, etymology_text supplied in this packet (kaikki:3f6f69a190036ae4:569092371)
From the picture
心情和臉色沉下來
這一家可以想成把同一個畫面搬到人身上。事情本身沒有變，變的是照在上面的那層光。希望少了一點，氣氛就沉了；表情上的光被遮住，臉就垮了。英文不換字，因為看到的變化是同一種。
verb · B2
If a mood, an atmosphere, or the feeling of a story darkens, it becomes less hopeful and more worried, sad or serious. You can also say that something darkens a mood in this way.
（心情、氣氛）沉下來
只活在一句老話裡
這一格要小心：它不是 darken 多出來的一個意思，而是一整句話的意思。不過畫面還是同一個——有人站在門口，身影會把門檻遮暗——所以它仍然掛在「光變少」這條線上。
verb · C1
Used to tell someone to go away and never come to your home again. The source marks it old-fashioned, and today people mostly say it as a joke rather than as a real order.
別再踏進我家門
verb · B1
To become dark, or to make something dark, because there is less light. It is used about the sky, a room, a stage, a screen, or anything that light is being taken away from.
變暗；使變暗
verb · B2
To make a colour deeper or stronger, or for a colour to become deeper on its own. The thing itself does not lose light; its colour changes.
（顏色）變深；加深