Is this only for objects (like "[that thing] is dark"), or can you say something like, "Det er mørkt ute" to mean "It's dark outside"?
Could "det er mørkt" be the answer to a dark joke? Like, "That is dark, man"
"Den var mørk." could work, referring back to the joke itself (masculine noun).
Note that dark/black humour is more commonly referred to as "svart humor" than "mørk humor" in Norwegian, though both work. The expression "galgenhumor" also exists, cf. gallows humour.
In northern norway when they say "det er mørkt ute" it means it is winter ;P
Can morkt also be used like dark is in English, to describe a dark story or movie?
You cant do that in duolingo, but it provides the letters. It's a shorthand for discussions