One thing that fans might have noticed with the My Hero Academia anime is that some fight scenes seem to inexplicably make their screens darken. Such occasions are most notable when fire is involved, as one would assume such a fight sequence would be much brighter and flashier.
However, the simple answer is that the screen darkens during these moments in order to protect the viewer. Several bright flashes might be realistic in some fights, but they would be terrible for people prone to epilepsy and other photosensitive disorders.
There are genuinely good reasons to avoid bright flashes in animes today, with the prime example being the Dennō Senshi Porygon incident in 1997.
The screen darkens in some My Hero Academia scenes for your safety
There are a few examples of the screen darkening in My Hero Academia, but this article will primarily focus on Deku vs. Todoroki in the U.A. Sports Festival. Take a gander at the above image. Some broadcasts featuring this fight will darken it to look like this.
This particular sequence between Deku and Todoroki seems much dimmer than one would assume, given that so much fire is involved. It's not as if Todoroki has some dark fire technique, so the naturally bright properties of his flames should look much different than what viewers actually see.
The screen brightens after this fight and returns to the usual My Hero Academia style that fans know and love.
The infamous Dennō Senshi Porygon incident
The Pokemon anime was extremely popular back in the day. Chances are, you're at least aware of the series since it's the most successful media franchise in the world right now. However, the important thing to note here is an episode called Dennō Senshi Porygon, which aired on December 16, 1997.
There were a few instances of flashing red and blue lights, with the most notable being when Pikachu used Thunderbolt on some missiles. The image posted above shows a still screenshot with that bright blue light shining pretty noticeably. Now imagine that section rapidly alternating between red and blue.
It's worth mentioning that 4.6 million households saw this episode, and 685 children were taken to the hospital. Also, it was reported that there were approximately 10% of viewers who suffered from mild symptoms in addition to that number.
Seizures, blindness, and other problems were all known to happen on that day.
Aftermath: Why some animes have the screen darken during combat
Pokemon and most other animes would abandon using bright lights in quick succession. Some shows, like My Hero Academia, would even darken the screen during some explosions during certain broadcasts to minimize potential risk.
It is worth mentioning that the Blue-Ray versions don't darken the screen as often compared to the original broadcasts. A viewer's screen darkening during explosions might annoy them, but it's ultimately for their health.