Whisk the blackberry puree and sugar in a medium saucepan over low heat; stir in the scraped vanilla seeds and add the bean. Stir and simmer mixture until sugar dissolves and is reduced to half cup, about 6 minutes. Remove from heat; pour into a small bowl, removing vanilla bean. Refrigerate for one hour or until cold.
Heat a grill to medium temperature. Spray grill top and both sides of the cut chilies with non stick cooking spray. Place chilies skin side down onto grill and cook until slightly blistered. Flip chilies over and grill second sided for 1-2 minutes, then remove chilies to a dish and allow to cool to the point where they can be peeled of skin. Meanwhile spray grill top again with non stick cooking spray and grill watermelon slices for 45 seconds on each side. Remove watermelon from grill to a dish. To assemble the salad, cut the chilies into strips; dust with ground cumin. Cut the watermelon slices into 1/2-inch strips and place onto large serving platter, alternating the red and yellow pieces. Arrange the chilies and red onion over watermelon. Sprinkle salt over top of the salad. Drizzle the coulis over all; scatter with shaved manchego cheese and garnish with blackberries. Pass extra coulis. Serves 4-6.