There are several synonyms for the phrase "become dark". One could say "turn dark" or "grow dark". Another option is "fade to black". "Dim" or "dull" can also be used to describe a fading or darkening of light. More specific synonyms include "shade over" or "shadow". Depending on the context, "cloud over" or "gloam" may also be appropriate. Overall, there are a variety of verbs and phrases that can be used to describe the process of something becoming dark, each with their own nuances and connotations.