Preheat oven to 350F, grease and line a 9×5” loaf pan.
In a small bowl melt the butter and set aside to cool.
Using a food processor or blender, blitz the lavender and 1 tbsp of sugar until combined. Stir this in with the remaining sugar in a large bowl.
Add the rest of the dry ingredients (flour, baking powder, salt) to the lavender sugar, and stir until well combined.
In a medium sized bowl whisk the eggs for 1-2 minutes, then add the rest of the wet ingredients (butter, vanilla, sour cream) & mix until combined well.
Pour wet ingredients into dry and fold together until the flour is just incorporated (if there are specks of flour that is OK, as they will be mixed in in the next step).
Toss blackberries with 1 tbsp of flour, then gently fold them into the batter until evenly distributed.
Pour batter into the lined loaf pan & bake in the oven for 50-55 minutes. It’s ready when a skewer comes out clean.
Let the loaf cool completely before glazing.