Our VN deals with some macabre themes centered around guilt, some mentions of blood, a couple injuries, but we never got to make any art or visuals for some of the things mentioned, so they're really just mentioned.

I'd suggest still adding content warnings for those. But instead of writing that the game features depictions of said themes, you could write that it features descriptions of them (that's at least how I personally handle content warnings of things that are only described but not depicted).