The Elves Club blue card is a magnetic card that identifies the members of the elves club. It allows the user to open electronic doors with blue locks.
When Twinsen frees Joe the Elf from the maze behind the seal of Sendell in the White Leaf Desert, Joe makes him the fourth member of the club. Twinsen can use the card to open doors and if he shows it to Raymond the Elf, he will give him five clover leaves.
"The Elves Club Blue Card. This card is coated with star powder and will allow you to open the doors with blue locks."