Mix dry ingredients together in a small bowl, ensuring baking powder and baking soda are well combined and there are no lumps in the mix.
Beat cream cheese till light and creamy. Add butter and beat till mix is again combined and creamy.
Add eggs, one at a time beating well in between so mix is still light and fluffy. Don't worry if the batter separates as it will come back together.
Add the sour cream and mix quickly till just combined.
Fold through the dry ingredients ensuring they are well distributed and combined but don't over mix the batter.
Very gently fold through the frozen or fresh blueberries, ensuring that the batter is minimally stained.
Fill a muffin or friand tray with muffin mix, smoothing off the tops.
Bake at 170 C/340 F for 15-18 minutes or till cooked through to the centre, taking care not to overcook.
You can poke about 4-5 blueberries into the muffin batter once the muffin trays have been filled with batter if you don't want to risk staining your batter. Press them down quite far and smooth batter over the top slightly to cover. Note that frozen berries can cause this mixture to stiffen so try not to clump them together. Slightly defrosting them can prevent this happening. Simply leave them at room temperature for about 1 hour prior to cooking.You may want to wipe them with a paper towel to remove excess juice. Store these in the fridge. They will firm up because of the cream cheese etc, so I suggest serving them at room temperature or warm them up to serve.
Recipe is suitable for freezing.