For Streusel topping, mix together both sugars, flour and cinnamon. Blend in butter until crumbly.
For Cake: Cream butter and sugar until light and fluffy. Beat in eggs, one at a time, and then vanilla.
Sift together flour, b. soda, b. powder and salt.
Stir together grated orange rind, buttermilk/yogurt and orange juice.
Add dry ingredients to butter mixture alternately with yogurt mixture, being careful not to overmix.
Gently fold in half of blueberries, then spread batter in 1 greased 13 x 9” and 1 greased 10 x 15” pan. Batter will be in a thin layer but rises a lot.
Scatter remaining berries over top, then sprinkle with streusel topping.
Bake at 350 F. about 40-45 minutes, or until tester inserted in centre comes out clean.
Serve with a dollop of vanilla yogurt or whipped cream.