Blue cards are required to access medium-tier puzzles in Rust, which provide better loot than basic puzzles and give access to advanced (red) puzzles. Blue cards can be obtained by solving basic (green) puzzles or by purchasing them from the tools vending machine at the Outpost (aka Compound) monument for 100 scrap. The following monuments have blue keycards:

• Sewer Branch
• Satellite Dish
• Airfield
• Arctic Research Base
• Military Tunnels
• Missile Silo
• Oil Rig
• Power Plant
• Trainyard
• Water Treatment

Large Oil Rig does not require a blue keycard to access the locked crate, but it grants access to 4 military crates. The Harbor is the best monument to get the blue keycard in Rust, and both the small and large Harbor have them. For more information on the locations of blue keycards, check out Rust tips and tricks videos on YouTube or Rust new player guides.