Blue Diamonds are a special currency obtainable through donating. You get one blue diamond for every $10 USD or equivalent money donated. Once the donation was processed successfully, the blue diamond shall arrive to your account, in your in game purse. In game, Blue Diamonds look like this. Once you got your Blue Diamonds, you can spend them at the Poporing NPC, located southeast from the mainland's city temple.
If you are far from the temple you can also open the blue diamond shop through using the blue diamonds, which will have the same effect as opening the shop on Poporing.
These Blue Diamonds are used to purchase a series of special cosmetics, such as wings and auras. The shop also has miscellaneous items such as boost scrolls and dungeon charges. Be aware that the cost varies from item to item.