Preheat oven to 350 degrees F. Grease muffin tin or line with cupcake liners.
In a microwave safe dish cover spinach leaves with water and microwave one minute (or use a small saucepan and simmer for about 2-3 minutes until spinach is starting to cook, don't over cook!).
Put the spinach leaves and a tablespoon or two of water into a blender and puree until smooth. Add yogurt and puree another few seconds until blended.
In a medium bowl, mash together butter and sugar with a wooden spoon until blended.
Add egg, lemon extract and lemon zest and beat until combined. Add spinach yogurt mixture and hand mix until combined.
In a separate bowl, whisk flour, baking powder, baking soda and salt. Combine with liquid ingredients until just combined.
Fold blueberries into the batter.
Pour into prepared muffin tin and bake for about 13-15 minutes (until toothpick inserted in the center of a muffin comes out clean.