Minecraft:Black entities

When an entity's origin point goes inside an opaque block and can be seen easily, they become black entities. This applies to mobs and some Minecraft:block entities.

However, if the block is an active light source, the entity doesn't get black.

Reason

File:Black painting beyond 524,288.png

Due to floating-point errors, this painting is black.

File:Black painting beyond 524,288 different effect.png

Ditto, but viewed at a different angle; notice how the origin point (center of painting) is inside the bone blocks.

Minecraft simulates Minecraft:light to determine how entities appear. Because light stops moving when it hits opaque blocks, entities with an origin point inside of an opaque block will become fully unlit and turn black.