Black decay does not appear to spawn.

[keybounce]

Black decay does not appear to spawn, no matter what instability level the age has.

Tested in a single biome deep ocean flat world (nothing but black ink from Y equals 62 down to Y equals 2), with instability levels for black ink set to 1000, 1000.

[XCompWiz]

Note that instability effects in an age are randomized, and it won't necessarily get every effect at any point of instability.

[keybounce]

Still, a deep ocean world of that much instability? I expected it to empty the decks.

How do you determine the actual instability effects chosen for the age/per deck/can you?

[XCompWiz]

I don't remember how it determines which worlds get what, but it probably did empty the decks for that world. Black decay may not have been in the world's decks.