Careful with these, they'll stain! Throw it at a player to turn him blue for an hour.
Blueberry is a consumable item. When used on yourself or other players, it adds the "Feelin' Blue" mod which remains active for an hour and turns your skin blue. When you use a blueberry on anyone there will be a small chance they will drop a blueberry seed, blueberry or gem. Farming Blueberry is not recommended as the Maximum Fruit Drop is 4.
You can create unique colors by using different skin color-changing items on a player. (Ex: Blueberry + Tomato = Purple. Blueberry + Egg = Grey. Mud Glob + Blueberry = Dark Blue) Experiment and have fun with them.
In 17 January 2016, the Blueberry Seed was re-textured and the colours have been switched, so the main seed colour is now blue and the spots are green, whereas the colours before were opposite.