A vibrant blueberry should have a little bounce to it. Shake the box when you buy them, if the berries do not move they may be getting a bit old.
The nutrients in blueberries are best sustained by keeping them cool, so purchase from a refrigerated section in the market and put them in your own fridge as soon as you get them home.
Select plump and full blueberries with a light gray-blue color. Blueberries with any hint of red are not fully ripened. Once blueberries are picked, they will not ripen any further.
To minimize streaking in cake or muffin batter, stir berries in directly from frozen.