The Bug

Potion particles from a mob/player can turn dark if inside a block, like suffocating mobs. It would be expected that these particles were normal-colored, or not show at all.

How to Reproduce

1. Make a small 1x2 hole in the ground. Make sure the blocks surrounding the hole are full solid blocks (not transparent blocks like glass).
2. Summon a mob inside it using the /summon command or a spawn egg, like a creeper.
3. Give the mob an effect using a splash potion or the /effect command (if using the latter, make sure to set hideParticles to false).
4. Note how some particles that go inside the blocks turn black.

Additional Notes

Related to MC-271499 and MC-271391.