Add all Blueberry Filling ingredients to a small sauce pan.
Mash blueberries with a potato masher.
Bring to a boil and stir frequently. Boil 2 minutes.
Remove from heat and allow to cool.
Pour through strainer, mashed it through with the back of a spoon, You'll have to scrap it off the bottom too. It's thick using the amounts listed above. For a thinner sauce (for pouring over ice cream, cake, etc) add 1/2 or more water. Sauce will get thicker as it cools. Cover and refrigerate 3 hours or up to 3 days for this recipe.
For the cake: Preheat oven to 325 degrees. Coat a 12 to 15 cup Bundt pan or tube pan with solid vegetable shortening the granulated sugar.
In a large bowl, beat butter, shortening, and Greek yogurt at medium speed with a mixer until combined. Gradually add the 3 cups sugar. Beat until fluffy. Add lemon zest and vanilla, beating until combined. Add eggs one at a time, beating after each addition. Stop the mixer and scrap the sides and bottom. Beat again.
In a medium bowl, combine flour, baking soda, and salt. Gradually add to butter mixture beating until just combined.
Spoon 4 cups batter into prepared pan. Using the back of a spoon, spread batter evenly around pan and flatten in the center.
Spoon 4 Tablespoons of blueberry filling into the center of the batter, using the back of a spoon spread the filling in a trench down the center of the batter. Top with 4 cups of batter, then another 4 T blueberry filling. Repeat the process until all batter and filling are used.
Bake for 1 hour and 20 minutes or until a wooden pick inserted into the cake comes out clean. Let cake cool for 20 to 30 minutes in a pan before removing. Remove from pan, allow to cool completely on a wire rack.
Whisk confectioners sugar and lemon juice until smooth. Drizzle over warm cake.