1. Fill and preheat the SousVide Supreme to 140F/60C.
2. Whisk the vinegar, sugar, salt, and 1⁄4 cup water in a medium bowl until sugar and salt dissolve.
3. Put the blueberries and onions into a small (quart/0.9 liter) zip-closure (or chamber vacuum seal) cooking pouch and pour the pickling liquid over them.
4. Use the displacement method (Archimedes principle) to remove the air and zip the seal (or vacuum seal in a chamber vacuum).
5. Submerge in the water oven to cook for 20 minutes.
6. Remove from the water oven and submerge the pouch in an ice water bath for 15 minutes. Use immediately or refrigerate for use within 2 weeks