Darkness fog was changed to be more like Blindness fog in 22w18a, but unlike Blindness, giving yourself a Darkness effect with a Duration value greater than 1638 makes the fog flash rapidly. It's about as severe of a photosensitive epilepsy trigger as I've ever seen so it should be patched as soon as possible.

Run the command below at your own risk!

/effect give @s minecraft:darkness 100000 0 true