The Blue Ball is an event item that can be unboxed from the Happy Blue Gift of Fun, a gift that was purchasable in Fancy Furnishings for $501 Money as a part of the 2016 Christmas Event.
Everything about this item is the exact same as its counterparts, the red and green ball, aside from its color.
The feature that makes these balls valuable is the number located on the ball. It is used as an indication on how many balls were opened by players in the game already, so if a ball has the number 386, then that ball is the 386th ball that has ever been unboxed in the game. Many players will tend to purchase the ball, as it may potentially have a special number to players that will drastically increase its value.