Blend the blueberries, erythritol (or sweetener of choice) and the yogurt until smooth.
If you wish to include chocolate chips in the smoothie, it's best to add them after the mixture is fully blended. This way, you won't have to blend the chocolate chips for an extended period, resulting in small chocolatey particles that add some texture.
Notes
Feel free to adjust the amount of sweetener in the recipe to your liking. The original recipe is on the less-sweet-side, but if you decide to add chocolate, it will be perfectly sweetened (in my opinion).
You can make it vegan by substituting the yogurt for soymilk kefir (the texture is runnier, but still delicious).
*The calories were calculated without adding chocolate.