For the rhubarb simple syrup: Put the sugar in a small saucepan with 1 cup water. Cook over medium-high heat, stirring occasionally, until the sugar starts to dissolve, about 5 minutes. Once it reaches a simmer, add the rhubarb and reduce the heat to medium low. Cook, stirring occasionally, until the rhubarb is softened and the syrup turns light pink, about 25 minutes. Set the rhubarb syrup aside until cool and slightly thickened, about 30 minutes.
For the strawberry fruit salad: Put the strawberries and blackberries in a serving bowl and drizzle generously with the rhubarb syrup. Top with the cooked rhubarb. Sprinkle with the basil and mint.