When it comes to synonyms for the word "dark substitute," there are several options to choose from. One possible alternative is "shadowy replacement." This phrase conveys a sense of something lurking in the background, waiting to take over when the original is absent or unavailable. Another synonym for "dark substitute" might be "dim stand-in," which suggests a lower level of brightness or clarity compared to the original. "Gloomy alternative" could also be used to describe a replacement that has a bleak or depressing quality, while "murky substitute" conveys a sense of ambiguity or indistinctness. Whatever alternative you choose, it's important to match the tone and context of the original phrase.