Personally, I do not see Batman Returns as all that dark. Yes, for time I recall the context and the complaints. Even after the first film in '92, people still expect something akin to Adam West. Penguin comes off as a poon hound and talks about blowing up children genitalia and what not. People die! Batman comes off as very devil-may-care when it comes to the lives of criminals. Part of it though is the cold, wintery palette though.
I must agree with Kevin Smith in that the film is kinda the beginning of the end into camp, NOT Schumacher. And I like how Glenn Weldon summed up the film in The Caped Crusade: Batman and the Rise of Nerd Culture as ‘aggressively silly’. If anything, it’s dark in the 90s, edgy sense of dark.