If the blueberries are mixed into the batter they are like little blueberry bombs that sink to the bottom of the cake tin and latch onto it. Nothing so disappointing than making a lovely bundt cake which looks like a slightly dilapidated sandcastle when it comes out of the tin. By putting them in the batter at the end they gradually drift down through the cake as it bakes and don't actually reach as far as the base. Also cut the bigger blueberries in half and give them all a roll in flour first.