In large bowl, whisk together flour, lemon zest, baking powder, baking soda and salt.
Whisk together eggs, brown sugar, yogurt, oil and lemon juice; pour over flour mixture, stirring until just a few streaks of flour remain.
Stir in blackberries just until combined.
Spoon into greased or paper-lined muffin cups.
Bake in 350°F (180°C) oven until tops are firm to the touch, about 25 minutes.
Let cool in pan on rack for 5 minutes; transfer to rack and let cool completely.
(Make-ahead: Store in airtight container for up to 3 days. Or individually wrap in plastic wrap and freeze in airtight container for up to 2 weeks.)
Glaze (optional): Whisk icing sugar with lemon juice; brush over muffins.
