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.