düster
gloomy
Very dark, poorly lit, or covered with dark clouds.
An dem düsteren Novembertag wurde es schon am Nachmittag dunkel.
On that gloomy November day, it was already getting dark in the afternoon.

gloomy
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gloomy
Very dark, poorly lit, or covered with dark clouds.
An dem düsteren Novembertag wurde es schon am Nachmittag dunkel.
On that gloomy November day, it was already getting dark in the afternoon.

The noun that 'düster' (gloomy/sinister) describes reveals the meaning. If it refers to light or a place, it means 'dark'. If it refers to feelings or abstract ideas, it means 'pessimistic'. If it refers to a person or their actions, it means 'creepy' or 'sinister'.
Wir gingen durch einen düsteren Korridor.
We walked through a gloomy corridor.
In the literal meaning 'dark', 'düster' (gloomy) is often used for places, the weather, or the time of day. Typical combinations are: ein düsterer Wald (a gloomy forest), eine düstere Gasse (a gloomy alley), ein düsterer Himmel (a gloomy sky), or ein düsterer Tag (a gloomy day).
Die Geschichte spielt in einer düsteren Burg im Mittelalter.
The story is set in a gloomy castle in the Middle Ages.
For the figurative meaning 'pessimistic', 'düster' (gloomy) is combined with abstract nouns. Common ones are: eine düstere Stimmung (a gloomy mood), düstere Gedanken (gloomy thoughts), eine düstere Prognose (a gloomy forecast), or düstere Aussichten (gloomy prospects).
Trotz der Krise hat er keine düsteren Gedanken.
Despite the crisis, he has no gloomy thoughts.
When 'düster' (sinister) describes a person or their character, it means 'creepy' or 'shady'. People then speak of eine düstere Gestalt (a sinister figure), eine düstere Vergangenheit (a sinister/dark past), or düstere Machenschaften (sinister dealings, plural).
Niemand wusste etwas über seine düstere Vergangenheit.
Nobody knew anything about his dark past.
black
'schwarz' and 'düster' can both describe something dark or negative. 'schwarz' is first of all a color: something is truly black, like night or coal. It can also figuratively stand for very negative thoughts or prospects. 'düster' more often describes low light, a sad or threatening mood, or an eerie effect. Learners may confuse the words because both are connected with darkness. For a color, 'schwarz' is more precise. For atmosphere, mood, or future prospects, 'düster' often sounds more natural.

Die Stimmung im Team war nach der schlechten Nachricht düster.
The mood in the team was gloomy after the bad news.
Nach dem Sturm war der Himmel fast schwarz.
After the storm, the sky was almost black.