Use fresh blueberries that are completely dry when you pop them in the freezer.
You can freeze the blueberries without rinsing if you want.  Just make sure to rinse them just before use.
If you prefer to rinse the blueberries first, dry them well with paper towels,
Once your blueberries are dry; line the cookie sheet or pan with plastic wrap (or other lining), then add one layer of blueberries. Try to see that they’re not touching; you want to freeze them individually. Put the pan on a level freezer shelf and freeze for 30 minutes.  This way the blueberries won't stick together and you can remove just the portion you need.
Remove the pan and place the berries into your container or plastic bag. Immediately return the blueberries to the freezer.
Blueberries will keep for up to 6 months in the freezer.