You can take the cover and paint or texture it to match the ceiling.

The other trick is to put a junction box FURTHER away in a less conspicuous spot and pull the wire to the new fixture and remove the old box entirely. If you're pulling parallel to the joists, you probably need only make a hole for the new box and fish all the way down. If you're pulling across the joists, you may have to cut more of the ceiling up, but at least you can replace that finished surface.

Thanks - putting the box in a more inconspicuous spot is a good suggestion I hadn't considered yet.

You can also put the box behind an access panel at a less conspicuous location.

The panel must use screws to hold it closed.

No nails allowed.