In a large skillet, heat the oil over a medium-high heat. Add the shallots and garlic and cook, stirring, for 1 minute. Add the blackberries and cook, stirring, for 2 minutes. Deglaze the pan with the vinegars and cook until reduced by half, about 30 seconds. Remove from the heat and transfer to a blender. Add the sugar, mint and tarragon, and process to a smooth paste. Add the salt and pepper. With the machine running, add the light olive oil through the feed tube and process until smooth. Strain through a fine mesh sieve into a clean bowl and refrigerate until completely cool, about 1 hour.
To make the salads, in a large bowl, combine the greens with the endive and pistachios and toss with the vinaigrette until lightly coated. Season, to taste, with salt and freshly ground black pepper.
To plate, divide the tossed greens among 4 plates and top with the goat cheese. Arrange 3 endive spears and the avocado slices decoratively around the edges, and serve. Garnish with extra blackberries.
Yield: 4 servings