Preheat the oven to 350º F. Grease or line a 12-cup muffin pan with muffin liners.
Combine the flour, baking powder, and salt together in a medium bowl and set aside.
In a separate large bowl, whisk oil, sugar, egg, and vanilla together.
Add buttermilk and whisk together.
Then stir in flour mixture until just combined.
Gently stir in the blackberries.
Divide batter between 11 of the prepared muffin tins.
Bake for 18-20 minutes until a toothpick comes out clean.
Remove from oven and let cool.
While the muffins are cooling, whisk the glaze ingredients together.
Drizzle glaze over the muffins.