They can be picked in the wild on Blueberry Bushes, before you can pick them you need to "Use" the bush. There should be only 3 berries left to pick. Still, sometimes other players "use" the Blueberry Bush and don't take them with themselves. Thus, there can be up to six blueberries in a bush. One hour is needed since the last player that used the bush for it to grow back the blueberries.
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After using Blueberry Bushes 500 times, you will earn the achievement Bluebarian.
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One Blueberry will regenerate hp/mana for 12 seconds (makes you 1% full).