Bring 1/2 cup water and 1/4 cup sugar to a boil in a small saucepan over medium heat, stirring occasionally, until the sugar dissolves. Let cool completely.
Combine the cooled syrup, peach nectar, orange juice concentrate, vodka and peach schnapps in a freezer-safe resealable container and freeze until firm but still scoopable, 8 hours or overnight.
Combine the blueberries, 1/4 cup water and the remaining 2 tablespoons sugar in a small saucepan. Bring to a simmer over medium heat, stirring occasionally. Cook, lightly smashing the berries as they soften, until the mixture is thick and jam-like, 5 to 8 minutes. Let cool completely, then press through a fine-mesh sieve into a bowl to remove the skins and seeds.
Pour the peach slush into tall glasses and top each with 1/2 cup seltzer; gently stir to combine. Add about 1 tablespoon blueberry sauce to each glass and gently swirl.