Preheat oven to 300 degrees.
In a large mixing bowl, combine the cream cheese, butter and margarine. Beat on medium-high for 3-5 minutes or until light and well blended. Add the granulated sugar and whip on medium-high until light and fluffy. Alternate adding the cake flour with the eggs. Beat just until the flour is incorporated and the egg yolks disappear before adding the next round.
Grease and flour (or use Baking Pam) a 12 cup Bundt pan or two large loaf pans. Add 2 tablespoons of cake flour to the blueberries and toss to coat. Add about half the batter to the pan and sprinkle with 1/3 of the blueberries. Cover with more batter, then more blueberries and repeat.
Bake for 1 hour 45 minutes at 300 degrees.
Cool 1/2 hour then remove to a platter and cool completely.
For the glaze combine 2 cups confectioners' sugar with the juice of 1 lemon or more until desired consistency. Drizzle over the cooled cake. Garnish with lemon zest if desired.