This salad makes a striking presentation — the jewel colors of the lettuces and blood oranges create a gorgeous effect, especially on white plates. The citrus-based vinaigrette is light and lively, with enough acid to let the berries shine. If your olive oil is very peppery, try substituting half the volume with walnut or hazelnut oil to reduce the "bite."
Make the vinaigrette dressing by placing the blood orange and lemon juices in the bottom of a large salad bowl. Add the sugar and salt, and stir until dissolved. Whisk in the olive oil.
Just before serving time, tear the lettuces into pieces and add to the bowl. Toss to coat the leaves with the vinaigrette. Divide the lettuce among 4 salad plates. Arrange some of the orange segments and blackberries on each of the salads. Sprinkle each serving with nuts and serve immediately.