Preheat oven to 350. Coat a 12-cup muffin tin with a cooking spray or line with paper baking cups.
In a large bowl, beat the sugar and the margarine with a wooden spoon. Stir in the yogurt, blueberries, yellow squash puree, egg, lemon extract, and lemon zest.
Add the flour, flaxseed meal, baking powder, baking soda, and salt. Stir just to combine, but do not overmix - the batter is supposed to be lumpy.
Divide the batter among the muffin cups. BAke until the tops of the muffins are lightly browned and a toothpick come out clean when inserted in the center, 13 to 16 minutes. Turn the muffins out onto a rack to cool.
Store in an airtight container at room temperature for up to 2 days. Or wrap individually and freeze for up to 1 month.