When effecting myself with blindness and night vision it wouldn't make the screen completely black, but instead will make the area around me that is visible brighter.
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The latest stance on this is that it was a bug, it was fixed in 1.15.2 - see here: MC-10480
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Was unable to reproduce; applying both effects still turns the screen black for me. As mentioned in MC-4647, the darkening behavior likely only affects non-NVIDIA graphics cards.