Step 1 In a small saucepan, whisk the water and sugar over high heat until the sugar is in suspension and no longer mounded on the bottom of the pan. Bring the mixture to a boil and cook until all of the sugar is dissolved, about 5 minutes. If this happens before the water boils, heat to the boil anyway.
Step 2 When the water comes to a boil, remove the pan from the heat and add the rose geranium leaves. Let the mixture steep at least 10 minutes while you're slicing the nectarines.
Step 3Combine the nectarines and blackberries in a large bowl or divide evenly among 6 small ones. Ladle the warm syrup through a strainer over the fruit. This can be done up to 2 hours in advance and held at room temperature until serving.