I thaw for that very reason...getting rid of the ice crystals and reducing the run off helps minimize the blue batter effect (we're always using frozen maine wild blueberries -- most recently from Trader Joe's -- for muffins and pancakes, and on top of steel cut oatmeal).

Also -- at one time, in a bout of blueberry zealotry, we overloaded some batter with frozen berries, so much so that muffins didn't bake in the time they should have. I chalked it up to too much of a thermal load.