In a medium bowl with hand mixer (or in a stand mixer), mix together cake mix, pudding mix, yogurt, water, eggs, oil, and lemon juice for 1-2 minutes, or until completely combined.
Pipe (or carefully spoon) mixture into greased donut pans, filling about 2/3 full. Bake for 12-14 minutes, until the tops of the donuts are light golden brown.
Allow donuts to cool, then dip in frosting and serve.
Blueberry Frosting
Place the blueberries, powdered sugar and lemon juice in a food processor and blend until smooth.
Transfer to a small mixing bowl and add the softened butter. Mix with a hand mixer until completely combined. Add more powdered sugar as needed to reach your desired frosting consistency (I added about another 1/2 cup).
Notes
Keep in an airtight container on the counter for 3-5 days.You could also freeze them in a freezer-safe container for 30 days. Thaw and eat as needed.