Minecraft wiki entry for Void, associated text: Entities are rendered and black below bedrock.
Looking at an old world from the old bedrock layer in Bedrock Edition.
The void as it looked in Alpha. Notice the blue color.
Bottom: Blue void from Alpha version.
The red Nether void.
A chunk error in an older Minecraft version revealing the void.
In 1.2.5, falling under Y=-618 would make a black box like this appear around the player.