Preheat oven to 350°F.
Generously spray the loaf pan with non-stick spray, line all sides with parchment paper, then spray another light layer of non-stick spray over the parchment paper.
Combine the flour, ¾ cup granulated sugar, zest of ½ an orange, baking powder, and baking soda, in a large mixing bowl.
In a microwave safe measuring cup, melt the butter by microwaving in 15 second intervals until melted.
Stir the milk into the melted butter with a fork, followed by the beaten egg, vanilla, and orange juice.
Make a well in the center of the dry ingredients and pour the wet ingredients into the well. Using a rubber spatula, fold the dry ingredients and the went ingredients until just combined.
Reserve about 15 whole blackberries, then very gently fold the remaining blackberries into the batter.
Spoon the batter evenly into the loaf pan. Gently even out the top then place the reserved blackberries on top of the loaf.
Bake for 60-65 minutes until the tops are golden brown and a toothpick inserted into the muffin comes out clean and dry. If the top of the loaf begins to get too brown, cover with aluminum foil for the remainder of the bake.
Towards the end of the baking time, combine the powdered sugar and orange juice and zest until a thick glaze is formed in a small bowl. The glaze should coat and very slowly drip off of the back of a spoon. If your glaze is too thick, add more orange juice or water 1 teaspoon at a time. If your glaze is too thin, add an additional tablespoon of powdered sugar until you reach the right consistency.
Once your loaf is out of the oven, using a fork, poke sets of holes about 1/2" apart all over the top of the loaf. Slowly pour the glaze over the top of the hot loaf. Allow to cool completely, then remove from the loaf pan, slice and enjoy.