Also called "Blue Chalcedony" and cabbed in similar manner as chryoprase, a type of green agate, the blue agates apparently are in form of veins with calcite impressions. It's not easy to find a specimen with a solid and whole banded pattern since many are hollow and in form of broken clunks. Some seem to have greenish color in their centers but I'm not sure if they're natural or enhanced.  The blue color is very likely due to optical illusions caused by water inclusions within the agates.