The Problems with Blacklists
They limit conversations
Blacklists don't just limit discussion of sensitive topics. People worry about slipping up and hurting someone, so they're very careful about what they say. A lot of topics aren't approached for fear of triggering someone, and even sending someone a cat picture can become a dicey affair if there's a piece of Swiss cheese in the background. What if someone is trypophobic and blows up at you over it? What if someone has a mental breakdown because of a picture of a bumblebee and blames you? One can be ejected from a community over a seemingly innocuous message because it contained the word "frog" (you may think this is an exaggeration; it's not, but I wish it were).
In communities with blacklists, it can be difficult to say anything because it could be a trigger. This is most obvious in communities with very long blacklists. No one can remember what is and isn't blacklisted, and checking every single message against the blacklist is both time-consuming and impractical. By the time you've decided your message is safe, the conversation has moved on.
They delegate responsibility for trauma reactions
Let's say someone is triggered by frogs. They enter a community and ask for frogs to be added to the blacklist. The moderators agree, and frogs now require a content warning. If someone posts a photo of a frog, this person can now blame the poster for their trauma reaction and potentially get the poster banned. Their trauma reaction becomes the poster's fault.
The person triggered by frogs is no longer held responsible for managing their own triggers and trauma reactions. Everyone else takes on that responsibility instead. Instead of expecting community members to be mature and manage their own reactions, everyone is expected to manage everyone else's reactions.
One might argue that it should be this way; avoiding someone's triggers should be basic respect. This is somewhat true. If someone asks you to stop sending frog photos because it distresses them, then you should probably stop sending frog photos while they're around. They set a boundary by asking you not to. It's an agreement between the two of you that would be rude to ignore.
Putting frogs on a blacklist isn't a personal agreement like this. Blacklisting is a global rule that obligates moderators to step in. Posting a frog isn't just rude; it's a punishable offence. Other community members may never meet the person triggered by frogs, yet if that person stumbles across a frog photo while casually browsing community posts, it becomes their fault for posting it (rather than something that the frog person should handle in private). Instead of setting a boundary, blacklisting shifts blame.
Even in communities with short and reasonable-looking blacklists, this can become a major problem. If a message even remotely falls under a blacklisted topic, then the person sending it is held responsible. The blacklist is short and sensible, so this seems fine until one realizes how subjective blacklists are. One server I passed through had "violence" on its blacklist. That seems reasonable enough, but the mod team stepped in when someone else sent a silly photo of a cat. The photo happened to have a kitchen knife in the background; moderation claimed that the sight of an ordinary knife could be triggering. The photo was deleted by mods and the person was publicly warned not to do it again. Nothing violent was happening in the photo, but the sight of a knife counted as a violent act to the mod team. This was a server aimed at mature adults who could handle their own boundaries, yet the blacklist was enforced to the extreme and no one was expected to handle their own reactions.
Content warnings don't actually help
All of the above could be discounted if content warnings had a significant positive impact. I figured I'd see what existing scientific backing content warnings have; if they were truly helpful and necessary, then there would probably be at least one study supporting them. Unfortunately for blacklists, the studies I found weren't in favor of their existence.
My exact search terms were "(content warnings OR trigger warnings) AND trauma" on EBSCOHost. This turned up two pages of results, a fair chunk of which I screened out for being heavily biased news articles. The remainder were clinical studies. One peer-reviewed study in the Clinical Psychological Science journal finds that warnings not only fail to have a positive effect, but also have the potential to worsen the effects of trauma. The Association for Psychological Science sums this up well:
"Specifically, we found that trigger warnings did not help trauma survivors brace themselves to face potentially upsetting content," said Payton Jones, a researcher at Harvard University and lead author on the study. "In some cases, they made things worse."
Worryingly, the researchers discovered that trigger warnings seem to increase the extent to which people see trauma as central to their identity, which can worsen the impact of posttraumatic stress disorder (PTSD) in the long run.
This search turned up another study along similar lines; it finds that providing content warnings does not help people brace for a trigger. It may also prolong processing of trauma memories by promoting avoidance. In other words, content warnings not only fail to prevent the negative impact of a trigger, but they may ensure that the trigger sticks around for longer.
One might argue that trigger warnings aren't supposed to help someone brace for a trigger. Maybe they're just allowing people to avoid triggers if they so choose. Unfortunately, avoiding the things that distress you doesn't help either. It may even make them more distressing by reducing tolerance. Avoiding triggers reinforces that you should avoid those things, which means they'll still be distressing in the future. This is a major part of why trauma stays traumatic. Avoiding the memories means they aren't being processed and integrated.
The only way out of the cycle is to face the trigger and re-teach your brain that it's not a threat. It might be necessary to face the trigger in small doses to do so, but the point stands that avoidance doesn't help. Avoiding triggers means they remain triggers. Content warnings encourage this avoidance and could slow recovery. There's a good reason why trauma isn't treated by avoiding what happened.
Interestingly, content warnings also reduce reading comprehension for traumatized students without improving reception of content. Not only do they fail to help, but they also worsen processing of the material.