Award Winning Blueberry Coffeecake

I've been making this coffeecake for YEARS.  It started with 4H demonstrations and sending it to the county and state fairs, and now I make it on special occasions (like Easter breakfast this year).

1/2 cup margarine (or butter)
1 cup sugar
1 teaspoon vanilla
3 eggs
1 cup light sour cream
2 cups flour
1 teaspoon baking soda
1 teaspoon salt
2 cups fresh blueberries
1 cup brown sugar
1 cup chopped walnuts or pecans
1 teaspoon cinnamon

In mixing bowl, beat margarine, sugar, and vanilla until light and fluffy.  Add eggs, one at a time, beating well after each addition.  Beat  in sour cream.
Sift together flour, soda, and salt.  Add to creamed mixture and beat well.  By hand, fold in blueberries.  Set aside.
In smal bowl, comine brown sugar, nuts, and cinnamon.
Spread almost half of the batter in greased 9x13 pan.  Sprinkle half of the brown sugar mixture evenly over batter.  Dollop remaining batter on top, gently spread.  Sprinkle remaining brown sugar mixture over batter.
Bake in preheated 350 degree ovenfor 45-50 minutes.

I say almost half of the batter because the batter is pretty thick, and super hard to spread over the brown sugar mixture.  Definitely easier to do if you have more batter to work with.  I last made this with half whole wheat flour and half white, and you can't tell at all.