Preheat oven to 425 degrees F (220 degrees C). Arrange asparagus on a large rimmed baking sheet; drizzle with 2 tablespoons oil and sprinkle with 1/4 teaspoon each salt and pepper; toss to coat. Spread into an even layer; roast until just tender, 8 to 12 minutes.
Meanwhile, for gastrique, whisk together sugar and water in a small heavy-bottomed saucepan. Cook over medium-high heat until sugar is dissolved, about 4 minutes. Continue to cook without stirring, swirling the pan gently to cook evenly, until the syrup reaches a light caramel color, about 5 minutes. Pour in vinegar (mixture will boil very rapidly and sugar will harden). Continue to cook, swirling occasionally, until sugar melts, about 3 minutes.
Add blackberries to saucepan; use a wooden spoon to gently mash. Cook until blackberries have broken down and sauce is slightly reduced and coats back of spoon, about 5 minutes. Remove from heat. Strain mixture through a fine-mesh sieve into a small bowl. Discard solids.
Pat scallops dry with paper towels. Season both sides with remaining 1/4 teaspoon each salt and pepper. Heat remaining 2 tablespoons oil in a large skillet over medium-high heat until hot and shimmering.
Working in 2 batches, cook scallops at least 1 inch apart in hot oil until scallops are golden-brown and release easily from skillet, about 2 minutes. Turn scallops and cook until golden brown, 2 to 3 minutes more.
Serve gastrique with asparagus and scallops. Serve with additional blackberries.
