From the Super Mario Wiki
The Card Key is an item found in Paper Mario: The Thousand-Year Door and Super Paper Mario. Card Keys are blue with purple triangles and a purple line, and the top is white. In Paper Mario: The Thousand-Year Door, it must be used to access certain areas in the X-Naut fortress on the moon. In Super Paper Mario, Mario must use it to get into the processing center in the Floro Caverns.