There are currently three different types of berries in Pokémon GO right now. Surely enough more are to be expected as the game grows.
The bluk berry could be used in Pokémon GO to double a number of stardust trainers receive or maybe even triple. Niantic probably didn’t expect a number of requests for a berry like this.
Stardust is needed more than candy because it can be used to power up any and all Pokémon. because the candies only works on the Pokémon of the same species. Not to mention more stardust is used in the process of powering Pokémon up.
If you are low on Stardust then this would make an excellent choice of berry to use when catching Pokémon to farm for Stardust. It’s always nice to try and get the most out of catching Pokémon.