Soak gelatin in 1/2 cup cold water in a small bowl 10 minutes.
Meanwhile, stir vodka, lemon juice, simple syrup, Campari, bitters, and 1 1/4 cups blood orange juice in a large measuring glass.
Heat gelatin mixture and remaining 1/2 cup blood orange juice in a small saucepan over low, stirring to dissolve gelatin; add to vodka mixture. Strain into 2-oz. plastic cups; chill until set, at least 6 hours and up to 5 days.
