Makes 18 Muffins
For the Crumble:
For the Muffins:
First, make the topping. Combine all crumble dry ingredients and then cut in the butter until the mixture resembles coarse crumbs. Chill for at least 1 hour. DO AHEAD: Can be made 3 days ahead. Cover and keep chilled.
To make the muffins; preheat oven to 325° Fahrenheit. Line standard 12-muffin tin with paper liners. In a medium bowl, whisk 1 cup all-purpose flour and next 4 ingredients. Set aside. Using an electric mixer, beat butter until pale and creamy, about 2 minutes. Add sugar and continue to beat until well incorporated, 2-3 minutes longer. Add the eggs and vanilla extract. Continue beating until light and fluffy, 3-4 minutes. Scrape the sides of the bowl. Add the buttermilk and lemon zest, gradually beating into the butter mixture. Finally, add the dry ingredients and beat until just blended (do not overmix).
Toss blackberries and thyme with 2 tablespoons flour in another small bowl; fold into batter, gently crushing berries slightly to release some juices. Divide the batter between prepared muffin pans (remember the recipe makes 18 so you will have to make another batch, we filled the muffin holes about 3/4 full). Top each muffin with 1-2 rounded tablespoons of crumble.
Bake until tops are golden brown and a tester comes out mostly clean when inserted in the center, about 30-40 minutes for standard-size muffins. Let cool in pan at least 20 minutes, then transfer muffins to a wire rack to cool. Serve warm or at room temperature.