Preheat oven to 375 degrees. Brush bread slices with olive oil. Arrange on baking sheet and bake until crisp and toasted, 8–10 minutes, turning once. Meanwhile, in medium non-stick skillet, heat blueberries, vinegar, and sugar just until blueberries are tender and a sauce forms, 4–5 minutes. Set aside and let cool. Top toasts with mascarpone cheese (about 2 teaspoons per toast) and sprinkle with kosher salt and freshly ground black pepper. Top with blueberry mixture and fresh basil.