Psyonix has published the drop rates for the loot in their Rocket League crates. According to the developer, the published figures have been true since the system went into effect in September 2016. Like most loot crate schemes, the rarer unlocks require a significant amount of luck to get. For example, the current Black Market tier of unlocks start with a 1% base roll to be present in the crate, and then each particular item in that tier has an equal chance of appearing. The good news is that you’ll probably unlock that common double-lightning decal pretty quickly.