finster
dark
Very dark; with no light or only very little light.
Als der Strom ausfiel, wurde es im ganzen Haus schlagartig finster.
When the power went out, the whole house suddenly became dark.

dark / sinister
Loading your saved state.
dark
Very dark; with no light or only very little light.
Als der Strom ausfiel, wurde es im ganzen Haus schlagartig finster.
When the power went out, the whole house suddenly became dark.

'Finster' (dark/sinister) is often stronger and more emotional than just 'dunkel' (dark). It describes a darkness or mood that is perceived as unpleasant, threatening, or absolute.
Durch die dicken Wolken wirkte der Tag finster und kalt.
Because of the thick clouds, the day seemed dark and cold.
In its literal meaning, 'finster' often describes places that are eerie or completely without light. Typical combinations are 'ein finsterer Wald' (a dark forest), 'eine finstere Gasse' (a dark alley), or 'ein finsterer Keller' (a dark cellar/basement).
Sie traute sich nicht, in den finsteren Keller hinabzusteigen.
She did not dare to go down into the dark basement.
To describe an unfriendly or angry facial expression, 'finster' is almost always combined with words like 'Miene' (expression), 'Gesicht' (face), or 'Blick' (look/glance).
Er warf mir einen finsteren Blick zu, als ich zu spät kam.
He gave me a dark look when I arrived late.
When it is about evil intentions or a bad character, 'finster' is used with nouns like 'Gestalten' (figures), 'Pläne' (plans), or 'Absichten' (intentions).
Im Krimi verfolgte der Detektiv zwei finstere Gestalten.
In the crime novel, the detective was following two sinister figures.
The phrase 'das finstere Mittelalter' (the Dark Middle Ages) is a fixed expression. It is used to characterize this era as backward and unenlightened.
Im Geschichtsunterricht sprachen wir über das sogenannte finstere Mittelalter.
In history class, we talked about the so-called Dark Middle Ages.
black
'schwarz' and 'finster' can both express darkness or something threatening. 'schwarz' is mainly a concrete color and can also figuratively describe very negative things. 'finster' usually means very dark because there is hardly any light. It can also describe an unfriendly expression, a threatening mood, or evil intentions. For the pure color, 'schwarz' is more normal. For rooms, nights, looks, or an eerie effect, 'finster' often fits better and sounds stronger.

Als das Licht ausging, war der Keller völlig finster.
When the light went out, the basement was completely dark.
Er kaufte sich für die Feier ein schwarzes Hemd.
He bought himself a black shirt for the celebration.