You can use unsweetened frozen blueberries instead of fresh blueberries in equal amounts. Don’t let them thaw; just put them directly from the freezer into your batter.
Do not over-stir the batter. The muffins will be too dense and tough.
Coating the blueberries in flour will help keep them suspended in the muffins while baking, so you will have a nice distribution throughout.
Oven temperatures vary and may need to be recalibrated periodically to ensure they are accurate. Make sure to check your muffins at the lower end of the recommended baking time.
Do not leave the muffins to cool in the muffin tin any longer than 5 minutes. The heat from the pan will overcook them.