Of course, the down side with dictionarys as opposed to people is that they tend to give the litteral translation and not the best word to be used 
Anyway, while in Dutch the word "donker" means dark, I am not sure that is the word you are looking for. The Dutch word "donker" is used mainly when there is a lack of light or when it is darker variation of a color. When you use "a dark night" to describe a somewhat threatening mood the word "duister" is probably much better. This especially true when you want the tranlation of darkness, because that would be "duisternis". When translating a word you should really give the context in which you want to use it.

Anyway, while in Dutch the word "donker" means dark, I am not sure that is the word you are looking for. The Dutch word "donker" is used mainly when there is a lack of light or when it is darker variation of a color. When you use "a dark night" to describe a somewhat threatening mood the word "duister" is probably much better. This especially true when you want the tranlation of darkness, because that would be "duisternis". When translating a word you should really give the context in which you want to use it.