Combine the brown sugar and water in a small saucepan and heat gently over low heat, stirring just until the sugar fully dissolves; do not boil. Remove from heat, add the blood orange zest and lemon zest, cover, and let steep for about 10 minutes to extract citrus oils without bitterness, then strain through a fine-mesh sieve and allow the syrup to cool completely.
Pour the cooled citrus syrup into a large pitcher, add the fresh blood orange juice and fresh lemon juice, 2 pinches of fine sea salt, and stir until evenly combined. Pour in the ice-cold water and mix gently until the drink is fully integrated and uniform.
Refrigerate for 1–2 hours until fully cold and the flavors settle into balance, stirring once before serving.
Fill glasses with ice, pour over the blood orange citrus drink, and garnish with blood orange slices and lemon slices.
