Cream together the sugar and butter. Then beat in eggs one at a time.
In another bowl, combine, lemon juice, sour cream, zest and vanilla. Then stir into sugar mixture.
In another bowl, mix together the flour, xanthan gum, baking powder, baking soda and salt. Then combine the two mixes by adding a 1/3 of each of the mixes alternately until blended thoroughly. Once the mixture is combined fold in the blueberries, then pour batter into a greased 9x13 pan.
Combine topping ingredients and scatter on top. Bake at 350°F for about 50 minutes. Cool in pan.