Psyonix has finally removed loot boxes from Rocket League and added Blueprints, which will provide players a specific loot, instead of random loots.
Rocket players can acquire random Blueprints at the end of matches and each Blueprint can build one item for a displayed number of Credits. If they like the item, they can spend money to craft it. Furthermore, the Blueprint can also be traded to a different player. You can check out the full patch notes here.