Which Black Market decals come out of which crates?

Meilzus your answer is close, but actually wrong. I posted this question a long time ago, and since then i've watched many unboxings. Here is the breakdown.

--- Black Market Items Released ---

Championship Crate 1: Labyrinth, Slipstream

Championship Crate 2: Heatwave, Parallax

Championship Crate 3: No New Black Market Decals

Championship Crate 4: Biomass, Hexed

Player's Choice Crate: No New Black Market Decals

Turbo Crate: Tora, 20xx

--- Current Possible Outcomes in Crates---

Championship Crate 1: Labyrinth, Slipstream, Heatwave, Parallax

Championship Crate 2: Labyrinth, Slipstream, Heatwave, Parallax

Championship Crate 3: Labyrinth, Slipstream, Heatwave, Parallax

Championship Crate 4: Labyrinth, Slipstream, Heatwave, Parallax, Biomass, Hexed

Player's Choice Crate: Labyrinth, Slipstream, Heatwave, Parallax, Biomass, Hexed

Turbo Crate: Labyrinth, Slipstream, Heatwave, Parallax, Biomass, Hexed, Tora, 20xx

Nitro Crate: Labyrinth, Slipstream, Heatwave, Parallax, Biomass, Hexed, Tora, 20xx

As you can see, for every new crate to get released, it's still possible to get older mystery decals in those crates. But it's not possible to get New Mystery Decals in older crates. The only exception to this, for whatever reason, mystery decals released in crate 2 could also be found in crate 1. I think after crate 4's introduction, they changed the equation to fit the model I've pointed out.

Per the Rocket League wiki http://rocketleague.wikia.com/wiki/Decals:

The Heatwave, Labyrinth, Parallax, and Slipstream decals can only be found in Champions Crate series 1, 2, and 3.

The Biomass and Hexed decals can only be found in Champions Crate 4, Player's Choice Crate 1 and Turbo Crate.'

The 20XX and Tora decals can only be found in Turbo Crate.

These are the guaranteed crates that they will be in. The rumor you heard about getting Heatwave from CC4 is likely false. Best of luck in your crate openings if you head down that route!