The Elves Club blue card is a magnetic card that identifies one as a member of the "Elves Club". It also allows one to open electronic doors with blue locks.
Confirming his membership in the club makes Twinsen eligible for a gift of 3 Clover Leaves whenever he meets an elf.
"The Elves Club Blue Card. This card is coated with star powder and will allow you to open the doors with blue locks."