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.

How to Reproduce

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.

Additional Notes

This is likely Won't Fix, due to the age of this bug.