Great question — answering as the UphillTech team:
There are numerous approaches. The simplest is paint. If a blank cover and its screws are painted to match the wall, the box will recede in appearance. Similarly, if the wall is wallpapered, you can wrap the blank cover with paper, gluing down a slight margin of the paper to the underside of the cover. Paint the heads of screws a color close to the paper's tone. You can buy medallions or rosettes (meant for centers of ceilings) that can be glued to a blank cover. If you search for ceiling medallions , you can find hundreds of styles, although many of them are pierced for electrical wires. Images and links are illustrative only, not an endorsement of goods or sources.
Full answer by bib: https://diy.stackexchange.com/a/97991 (CC BY-SA 4.0).
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