Preheat oven to 350°F (175°C). Grease a 9x5-inch loaf pan, line with parchment paper leaving overhang on two sides, and lightly spray the parchment.
In a large bowl, whisk together the flour, sugar, baking soda, salt, and cinnamon for about 30 seconds until evenly combined.
In a medium bowl, whisk together the buttermilk, eggs, vegetable oil, and vanilla extract for about 1 minute until smooth and slightly frothy.
Pour the wet ingredients into the dry ingredients and gently stir with a spatula just until no dry flour streaks remain, about 15–20 strokes; the batter should still look slightly lumpy.
Add blackberries (frozen berries go in straight from the freezer; fresh berries should be completely dry) and gently fold into the batter with 5–8 strokes, keeping berries as whole as possible.
Pour batter into the prepared pan, smooth the top, tap the pan on the counter 2–3 times, and bake on the center rack for 60–70 minutes, tenting with foil if the top browns too quickly, until a toothpick inserted in the center comes out with just a few moist crumbs.
Cool in the pan on a wire rack for 15 minutes, then use the parchment handles to lift the bread out and cool for another 30–45 minutes before glazing or slicing.
Whisk together powdered sugar, vanilla, and 2–3 tablespoons of milk until smooth and drizzleable, then spoon over the cooled bread and let set for 10–15 minutes before slicing.