To make the grenadine, combine the pomegranate juice and sugar in a small saucepan over medium heat. Stir until the sugar is fully dissolved, about 5 minutes. Do not boil, but continue cooking until syrup is reduced by at least half, about 30 minutes.
Remove from heat and stir in rose blossom water. Cool completely before using, about 30 minutes. Store refrigerated in a sealed jar for up to 3 weeks.
Once the grenadine is cooled and you’re ready to build the cocktail, fill a cocktail shaker with ice. Add the rum, lime juice, and bitters.
Put the lid on the shaker and shake hard for 15 seconds.
Strain into rocks glasses filled with fresh ice.
Top with blood orange soda and gently stir.
Slowly pour grenadine down the inside edge of each glass to create a gradient effect. Do not stir.
Garnish with a blood orange slice and edible flowers, if desired. Serve the rum sunset cocktails right away.