If your using a specific amount of water in the soup, just ladle some out of the soup into the blender with the beans , then dump it all back into the soup after blended. This way the proportions won't change.
That being said, the blended beans will add a thickness to the soup, so you may need to add a little extra water to the recipe to get the consistency you are looking for.
Sometimes soups with just water taste a little empty to me, so I add a vegetable bouillon, or vegetable broth in place of the water for a fuller taste.
If your dietary rules allow, a sprinkle of parmesan at the end when eating will also fill up the flavor, or , sometimes i even blend a little parmesan up with the beans so the entire broth is infused with the parmesan taste ( or just toss a parmesan rind in the soup as its cooking).