Blend all ingredients with a hand blender (or in a food processor) and let batter rest for 20 minutes. If you are whisking by hand, strain batter before resting.
To make crepes, heat a large non-stick pan over medium heat. Spray with food release, or lightly coat with oil (you will only have to do this every other time).
Pour 2 ounces of batter into the center of the pan and lift the pan off the heat, swirling the batter around to coat the pan evenly. Return pan to heat and cook crepes for 3 minutes, until edges turn light brown. Gently loosen edges of crepe with a spatula and flip crepe over in pan. If this presents a challenge, simply turn burner down to medium low heat and cook crepe longer on one side, until crepe is completely dry looking (crepe will cook without flipping). Loosen edges of crepe and slide onto a plate to cool. Repeat with remaining batter.
After crepes have cooled, be sure to wrap well until ready to fill. Do not store in fridge - if not using right away, freeze and thaw when ready to fill.
Stir together ricotta, sour cream and sugar. Stir in egg, egg yolk, vanilla and nutmeg.
Butter a baking pan with melted butter. Place 2 crepes on the counter, with the paler sides facing up. Place about 2 tablespoons fresh blueberries in the center of each crepe and spoon about 3 ounces of the ricotta filling on top of the berries. Fold edges of crepe over 4 times to create a square parcel. Turn parcel over and place in baking pan. Repeat with remaining crepes, placing them very close to each other in the pan.
Brush tops of blintzes with butter, and sprinkle with a little sugar. Bake for 15-20 minutes, until they puff up like little pillows.
Serve immediately, dusted generously with icing sugar and with a little blueberry sauce or fresh berries. A scoop of vanilla ice cream also has a place on such a dessert!
The blintz crepes freeze wonderfully! Similar to classic French crepes, blintz crepes are more tender due to the great amount of butter in them.