I think they mean virtually the same thing, given that they're being used similarly when you browse through the posts while the latter has a better name than the former. Rather this, the reverse alias or nuking one of the two tags because looks like padding.
I've been using black_background when there's a solid black color on the background and dark or dark background when it's a more complex dark background rather than just solid black. This has also been discussed on forum #111514.
I've been using black_background when there's a solid black color on the background and dark or dark background when it's a more complex dark background rather than just solid black. This has also been discussed on forum #111514.
It seems the wiki definition of Dark is aligned with what was discussed back in topic #111514. However I'm not sure what would be the best tag name for this concept, because virtually all "___ background" tags are very related to simple background and we know dark background doesn't mean it's a simple bg. However, dark as tag can be very ambiguous to a lot of different meanings.
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.
This seems to be a rather broad use which make even more clear it's an ambiguous tag, I honestly don't know how I'd tag the two last examples you gave though, they definitely don't fit neither black background nor dark background, specially post #4338895. But I still believe such cases deserves a better tag than simply "dark", if it doesn't exist already. Even if the tag continues to exist just for such specific cases, we'd still have dark and dark background overlapping in a lot a posts while we don't need two tags to express virtually the same thing.