If "mörke" is used the same way in Norwegian as "mörka" in Swedish, then no, it does not mean the same thing as murky. (murky is translated as "grumlig" in Swedish, would be interesting if someone could say what it is in Norwegian).
However, it is very possible that mörk and murk stem from the same root, since both imply something that you can't see through :)
Adding a "t" is the correct rule. "Mørke" is the plural form, however: The word stem, to which you should add it, is "mørk". Thus we get "et mørkt skjerf" (neuter; "a dark scarf"), "en mørk mann" (masculine; "a dark man"), "en/ei mørk dame" (feminine; "a dark woman") and "mange mørke skjerf" (plural; "many dark scarves").