I often recommend freezing the berries first before drying them.  Blueberries have a thick skin so they recommend dipping fresh berries boiling water for 15 seconds and then in ice water to crack the skin OR you can freeze them first to soften the skin.  
Just put them on your dehydrator trays frozen and dry.  135-145 degree temperature until they are shriveled and leathery and no longer moist inside.

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Nellie Oehler