The Bug

The death particles that emit from a mob once its 'corpse' disappears can sometimes be black instead of white when inside of a block (like suffocating mobs). See the image attached for a visual example.

It would be expected that the particles would either be white or not show at all if inside a block.

How to Reproduce

Note: In my testing sometimes the corpse would escape the block and display normal white particles. Keep trying this method until you get the black particles to show, as shown in the video.

Dig one block down and inside that hole and spawn a mob (preferably a two-block tall mob such as a creeper).

Run the command below to place a block in the mob (or use any block, just make sure it is a full non-transparent block):

/setblock ~ ~ ~ minecraft:stone

Optionally run the command below to quickly kill the mob:

/effect give @s minecraft:strength 30 9 true

Kill the mob while it is in the block. Note how some particles emitted are black due to them originating in the solid block.