Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: Blackfish
For a documentary about Seaworld, this film has a crapton of Nightmare Fuel, even in the trailers!
In the trailer alone, we hear a couple of phone calls from SeaWorld staff to local police. One call tells how Tilikum (the orca who is the focus of the documentary) has trapped a trainer underwater. The second call states that Tilikum hasn't just drowned a trainer, but eaten her.
The poster◊ for the film is plenty scary, showing an orca facing front on a black background. The only color bright enough to see being the large white patches that resemble eyes at the front of his head.
Tilikum himself is a frightening film subject. When he was brought to SeaWorld, he was twice the size of the other whales at the time. As he grew up, he was also quite well known for injuring trainers. The documentary covers what happens when he finally killed them.
Much of the footage of Tilikum is Nightmare Fuel, showing how he injured trainers and eventually killed Dawn Brancheau (Senior Trainer at SeaWorld for years).
The detail to which Dawn Brancheau's body was savaged, and the trainer in Spain who was rammed by an orca is described by his mother and fiance as appearing as though his chest exploded. To make matters worse, when the fiance recieved the initial phone, she was told "He's fine."
Ken Peter's incident with a killer whale (specifically Kasatka) has him being dragged down to the bottom of the tank multiple times. Possibly the only reason he survived was because he was an experienced scuba diver. Though, that's not all. The other trainers put a net out. When the whale finally let go, Ken "swims like a demon" to get out of the tank. Then the whale goes over the net. Ken manages to get to safety though, albeit with damaged feet.
It's especially scary if the viewer knows that orcas, at least in the wild, typically won't try to go over nets even when they easily can (hence why people trying to catch orcas in the wild can trap a pod by surrounding it with a net).
The way he quickly breathes before being dragged back into the water again by the orca. What's scary is that Ken knew that he was going to be dragged under for a third time and was aware that he couldn't do anything about it but wait for the right time to escape.
The bloody scratches Tilikum gets from being bullied by the female whales.
Speaking of those bloody scratches, they are called "rakes" and happen when orcas scrape at each other with their teeth. Footage in the film shows one of the SeaWorld orcas going onto the slideout with a particularly nasty rake mark that is just gushing blood.
The bloody battle between Kandu V and Corky. The two orcas get into a tussle during a show. Kandu V rams Corky only to sever an artery in her jaw when she misfires and ends up hitting the far wall of the tank. She bled out for 45 minutes and footage of this exists, with blood spouting from Kandu V's blowhole and finally ending with Kandu V lifeless in the tank with her baby Orkid swimming around her. Footage here.
The film of the trainer who is dragged into a tank and only saved from certain death when another trainer thinks to let in a more dominant whale into the enclosure is terrifying.
The use of actual footage, be it of Tilikum's behavior, news reactions, SeaWorld capture facilities... it's disturbing because you know it's real.
The guy who gets crushed between two whales and winds up hospitalized. What makes it worse is that from the video, it's ambiguous whether the whale decided to be aggressive or just jumped at the wrong time. Even when the whale's not actively trying to hurt you, you could still end up paralyzed from it missing its cue.
The death of Keltie Byrne. The 20-year-old only had a part-time job at the Sealand park and was intending to keep swimming competitively. She ended up drowning due to the whales (specifically Tilikum) pulling her underwater despite her attempts to escape, and the eyewitness accounts make it all the more horrifying: the reason she couldn't get enough air to stay alive underwater was because she was screaming for help while she was above water and at one point one heard her say, "I don't want to die!" The other workers couldn't save her because the whales would drag her away whenever she tried to swim for a life preserver. And what lead to this horrifying situation? She simply slipped after feeding the whales and was dragged away as she was trying to get out.
The fact that the details of the deaths and injuries of the trainers were allegedly covered up by SeaWorld so that the trainers would be responsible, not SeaWorld itself for its policies. This is to the point where officials came out directly saying that the trainers were at fault for the incidents, at one point stating that Brancheau herself would agree that she would be at fault. Imagine if the blame of your death was coldly placed on you so that a corporation wouldn't have to make changes to abusive policies and discrediting anyone who stated that you weren't to blame.