Watermelon and Blue Cheese Salad

½ cup light olive oil

¼ cup white balsamic vinegar

salt and ground black pepper to taste

5 pounds watermelon

cut into bite-size cubes

1 small sweet onion (such as Vidalia®)

thinly sliced

2 tablespoons chopped fresh mint

or more to taste

6 ounces crumbled blue cheese