from CRC: (regularly updated)
Blueberries, fresh
Agitate the blueberries in soapy water for a minute, and then agitate one more time in plain water. This does not apply to organic and u-pick blueberries which require the thrip cloth method.
Blueberries, frozen Acceptable Without Certification
Frozen blueberries, without any kosher sensitive ingredients added, are acceptable without certification. Organic frozen blueberries are only recommended if being pureed.