Place your blackberries in a single layer on a sheet pan and place in the freezer at least 15 minutes, up to 3 months in advance.
Preheat oven to 425°F. Spray the wells of your muffin tin with nonstick spray or line with paper liners and lightly spray them.
In a medium mixing bowl, whisk together your dry ingredients. Add one tablespoon of the mixture to your frozen blackberries and toss together. Set into the freezer.
Melt your butter and allow to cool down while you make your batter.
In a large mixing bowl, combine buttermilk, eggs, and almond extract. Continue whisking while adding in melted butter.
Fold in the dry ingredients, just until the last of it disappears. Fold in the frozen blackberries, just until evenly dispersed in the batter.
Use a medium scoop, or a couple spoons, to divide the muffin batter evenly into the the prepared muffin pan wells. They will be quite full. Top with sliced almonds. Add sparkling or granulated sugar to the tops, if desired.
Bake at 425°F for 5 minutes, then drop the temperature to 350°F, without opening the oven. Bake for another 18 to 22 minutes, or until a toothpick or skewer inserted into the center of a muffin comes out clean or with just a few crumbs.
Let cool for 5 minutes in the muffin pan and then carefully transfer to a wire rack to cool for at least 15 minutes longer.