The background here is clearly not suitable by analogy with other tags.
The tags discussed are now ambiguous, both due to the simplicity of the word in general terms, and the lack of a description for the "background" tag.
dark > post #4544558 implies an analogue of darkness or even an emotional component; post #4440169 some blackout; post #4337186 post #4338895 also does not fit the description of the tag.
dark_background analogue of black background. post #5098285 post #4991225 misuse; post #5009768 post #4990938 direct analogue.
For images without a clear light source that cannot be directly attributed to night, I would look at replacing with "blackness". Or other analogues, similar in meaning and understandable to a wide range of users. https://www.merriam-webster.com/thesaurus/semidarkness
This includes images with a dark environment, but without a solid black background. It can also include places in the shade.
However, the absence of a material environment (floor, walls, earth, etc.) should be marked with a dark background.
The discussed tag "dark" is better to transfer to the category of ambiguous.