One of the first things you are introduced to in the game is the Stamp Card, which is given to you as incentive to basically do the sidequests. The majority of the sidequests and some of the main quests will reward you with a Stamp upon completing them. There are different Stamp Cards, with Black being the fourth, and final, one you get. In order to fill it up, you will need to collect 30 Stamps, in addition to the 30 to fill up the Silver card and 50 Stamps each for Gold and Platinum. Note that in order to complete this, you will need to have finished the main story, as you won’t have enough to fill it with all of the quests beforehand.