The hit glow can be removed if you like the dark look of the gameplay trailers.

Something I pointed out when we discussed Diablo 4’s figurative and literal ‘darkness’ back in 2019 is that, although you can’t see it in the official gameplay trailers, hitting them makes them flash with whitish light. That’s a pretty normal videogame thing, but it does somewhat undermine the sense of being swallowed by darkness. Good news if you’re not into it: You can turn it off.
During a BlizzCon Q&A stream today, game director Luis Barriga said that Diablo 4 will include a suite of accessibility options, and that the hit glow will be one of the available toggles.
“So, we really like how [the glow] works—the emphasis that it places on how visceral combat feels—but we have run into situations where specific members of the team find it very distracting, almost an impediment to playing the game,” said Barriga. “So, we do have it as an option as part of our accessibility suite. We do want to be sure that our game can be played by as many people as possible.”
Separately, I asked Barriga if it was also possible to turn off the glowing red outline that appears around enemies when you move the cursor over them. He said there’s nothing to announce about that right now, and that he was only referring to the hit flashes in the Q&A. So, that’s unknown for now.
To see the hit glow I’m talking about, you have to watch some of the gameplay captured by press during BlizzCon 2019, because the official Diablo 4 trailers leave out both the red outlines and the hit glow. I’ve embedded Game Informer’s 2019 video below so you can see what I mean.
I think it’s fair to guess that these informative lighting effects are omitted from the official trailers because a certain amount of coolness is lost when ‘videogamey’ elements show up to help with visual clarity. I do think the game looks better without the red outline or white flashes, although there’s a pretty good chance I’ll end up agreeing with Barriga and keeping the glow on so that I can see what the hell I’m hitting in those gloomy crypts. (I’m just assuming I’m going to play Diablo 4 because, well, I’ve played the other Diablos, and it feels a bit inevitable.) 
Whichever setting I end up preferring, I’m glad to have the option to try playing with a plainer, less modern look. I’m also curious to find out what else is included among those accessibility options, as I always hope for a high degree of tweakability. 
In other Diablo 4 news, the new trailer for BlizzCon 2021 shows off the Rogue class, which we’ve got a bunch of details on here. We also learned how Diablo 4’s open world PvP will work. 
One thing we didn’t get at this year’s BlizzCon is a release date for Diablo 4. We didn’t really expect one, though. It won’t be out this year, and I’m not sure it’ll be out next year, either. Back when Diablo 4 was first revealed in 2019, Barriga said that it could be a bit of a wait. In the meantime, here’s what we know about it.