Select plump, full blueberries with a light gray-blue color. A berry with any hint of red isn't fully ripened. White and green colored blueberries will not ripen after they are picked; while blueberries that have already turned purple, red or blue-ish usually DO ripen after they are picked (if they are kept at room temperature to ripen).
Since blueberries hang on the bushes in bunches a lot like grapes do, the easiest and fastest way to pick them is hold your bucket under them in one hand and with your other hand, cup a ripe bunch and gently rub them with your fingers. The ripe berries will drop into your bucket, while the unripe ones will remain attached to the bush.
When the bushes are at peak, I can easily pick 2 gallons per hour (if I'm not being distracted by the kids and the sun isn't too hot!). A newbie might do 1 gallon per hour.and at the beginnng or end of the season it takes more time as the berries are not as plentiful nor concentrated in clusters.
Keep in mind that blueberries vary considerably in density and moisture content, so these ranges are approximates.
1 gallon of blueberries weighs about 7.5 lbs or (4 liters of blueberries is about 3.5 kg)
1 pint of fresh blueberries weights about 3/4 of a pound. (1 liter of blueberries is about 700 grams)
1 pound of fresh blueberries is usually between about 2 and 3 cups of berries.
It takes about 4 cups (about of blueberries to make a blueberry pie
A normal batch of blueberry preserves, jam or jelly requires 5 pints of berries.
Blueberries do come in a variety of sizes from small (190-250 berries per cup) to extra large (<90 berries per cup)
Here are a few good links: