Heat grill to medium-high. Rub coriander over steak. Toss blackberries with 1/2 teaspoon oil on two sheets of foil; seal to form a packet.
Generously season steak with salt and pepper. Grill steak, corn, and blackberries, turning steak and corn occasionally, for 10 to 12 minutes for medium-rare steaks, charred corn, and tender berries. Let steak rest while preparing salad.
Let the berries cool in their juices. Cut kernels off corn cobs. Whisk soy sauce, lemon juice, remaining 3 tablespoons oil, and juices from the berry packet in a large bowl. Add endive, onion, greens, corn, and berries; toss well. Divide among serving plates.
Cut steak along grain into 3-inch-wide strips, then cut against grain into 1/2-inch slices. Scatter over salads and drizzle any meat juices on top.
Wine Pairing: Pinot Noir