1) Preheat your oven to 375°F. Grease a 9" x 13" pan, or two 8" X 2" round cake pans.
2) To make the topping: Combine the soft butter, brown sugar, 6 tablespoons muffin mix, pecans and salt and stir until the mixture is crumbly. Set aside while you prepare the cake batter.
3) Mix the batter, starting with the remainder of the muffin mix, butter or oil and eggs, on medium-high speed until well-blended.
4) Add the milk, 1/4 cup at a time, blending well and scraping the sides of the bowl between additions. When all of the milk is added, mix on medium-high speed for about a minute or until the batter has thickened. Gently fold in 1/2 cup of the blueberries.
5) Pour the batter into the prepared pan(s)and allow to rest for 10 minutes. Top with the rest of the blueberries, then the streusel topping.
6) Bake the cake for 35 to 45 minutes, or until a thermometer inserted into the middle reads 210°F. Remove it from the oven, and cool slightly before cutting.
Yield: about 16 servings.