In English as Germanic language K is ignored at the beginning of word in speech. Night and knight have to be pronounced similar. Then how to differentiate?
"The Black Knight" isn't going to provide enough context to determine which (k)night is meant; "The black knight rode in on his charger" probably will.
The presence of an article might help; knight will usually need some determiner and night often does not. However you will still encounter a sentence like "the night was dark" where you have to rely on further information.
[Incidentally, Knecht in German does have its K pronounced, or certainly did when I was taught German.]
If you hear "It was at night/during the night" you won't hardly think of "knight". The structures in which those two nouns are used are totally different. If in a language two words are pronounced alike you may assume that the structures they are used in are different. Otherwise the community of speakers would differentiate them.