I agree that the quilt wants a little brightening, but I'm not sure trying to brighten it with a light border will work. That might just make the interior seem all that much darker. It may seem counterintuitive, but adding a darker border, perhaps a warm brown print a bit darker than the sashing, might make the interior seem brighter/lighter by contrast. If you prefer a blue, I would choose something less bright than the star print. I would pin/lay a bunch of fabrics in varying values beside the quilt and take photos, to see what effect various border values have on the whole.