Fresh and frozen blueberries will have a tendency to sink down into the batter while baking. A simple solution to prevent that is by lightly coating the berries with flour before adding to your batter. You can do the same with pancake batter. Just don't add too much extra flour. If you want your berries evenly distributed throughout the loaf, pour just the batter (without berries) into the loaf pan, then use a bamboo skewer or long toothpick and gently press the berries into the batter everywhere you want them. I would still dust them with flour first.