Too dark is when the world tells you that "enough is enough"
Okay think about this, "shock factor" yes it is indeed a thing and it's pretty common on the internet and wherever else it's found but there's a reason why there are rules put up in accordance with such depraved, down under deep dark deep down abyssal type of messed up context. In my opinion, too dark is when you just put it there because
1. It's no longer relevant in the story yet it still added in as a pointless filler.
2. Just begging to hope that the shock factor can at least carry with the plot instead of something else, something better.
3. The story is entirely dependent on "too dark" blob of texts.
As far as details go, best to go with the most simple implication and try to think how it makes a reader in their perspective think, imagine, visualize the intended imagery based on what was read from that particular scene. Predict it but know that every reader is an individual, no same person can react alike like another unless there's a simple understandable agreeable circumstance that you put the character/s in the situation.
Aight I'm done brainstorming, hope this helps.