Preheat the oven to 175°C (350F) and line an 8x4" loaf pan with parchment paper.
Sift and mix the dry ingredients (flour, cocoa powder, salt, baking powder, baking soda) and set aside.
Place sugar and butter in a large mixing bowl of a stand mixer fitted with the paddle attachment. Mix on medium speed for 1-2 minutes until pale and fluffy.
Add the egg and vanilla extract and mix for another 2 minutes until well-combined. Add the blood orange juice and zest and mix for another 30 seconds. The mixture may curdle slightly - this is normal.
Slowly add the dry ingredients in two additions, alternating with the milk and mix on low speed until just combined. Do not overmix.
Pour batter into prepared loaf pan and bake for 45-60 minute or until a skewer inserted into the center comes out clean. Let the cake cool for 5 minutes in the loaf pan, then transfer to a wire rack to cool completely before glazing.
To make the glaze, stir in 2-4 tbsp. of blood orange juice into powdered sugar until the desired consistency is reached. For a thinner glaze, add more juice. For a thicker glaze, add more powdered sugar. Drizzle over cooled cake, leave to set for 20-30 minutes and serve.