Make the topping first and set it aside, so you can drop it right in. Mix everything together in a bowl with a wire whisk, until the lumps made by the butter are no longer visible. (It helps to use softened butter.)
In a mixer bowl, cream the butter and sugar together until it's no longer gritty. Stir in the zest, vanilla, and almond extract. Add a third of the flour and beat in. Add an egg and beat in. Repeat with another third of the flour, another eggs, the rest of the flour, and the last egg.
With a rubber spatula, mix four cups of the blueberries into the batter. It's okay if you break a few, but don't be too rough.
Grease a ten-inch cake pan. With the spatula, spread the batter uniformly in the pan. Top with the remaining blueberries. Finally, spread the topping uniformly over the blueberries.
Bake in the center of a preheated 350-degree oven until a medium brown. Test with a skewer or toothpick after 45 minutes; if it comes out not coated with batter, it's ready. If not, give it another five or ten minutes.
Cool in the pan on a cake rack for about half an hour. (An hour is even better. This doesn't need to be served warm.)
