Or a loose connection is arcing slightly and producing ozone gas

No. It's absolutely the next penicillin

The nearly perfectly symmetrical shape makes me think its due to the electromagnetic fields when current is passing through the socket to whatever is plugged in, but I have no idea why such a small amount would have such a dramatic effect on the mold.

Yeah, I think this is a lot more plausible than the wall heating theory that someone further up got way more upvotes for.

Most likely.

The box tends to sit slightly proud of the wall. When they plaster it in (particularly retroactively) they need to build up the plaster around the socket.

Thicker plaster means more insulation, and less condensation. Less condensation, less mold.

The ring around the socket also supports this. The edge where the new plaster gives way to the old. That point tends to be rougher, and so more prone to capturing mold.

It could also be thicker paint causing the same effect. Most people do the wall with rollers, but sockets with a brush, leading to a thicker coat.

Fairy ring?

Damn, that's beautiful. In German they're called a witch Ring

Stick conductive metal in the outlet and you will be transported to the fey realm.

TIL that it is not just something in RuneScape

Mushroom circles grow in circles because they drain the nutrients from the ground, and eventually you have a nutrient barren center that grows with the ring of mushrooms around it.

I wonder if this is something similar, although I've not heard of mold using up a resource before.

Wait, people like the UK socket?

Thousands of species of mold appear black (actually dark green). The one that is usually singled out in this made up category is Stachybotrys chartarum. The whole “black mold” thing is the result of several irresponsible people who are drumming up fears about mold and then profiting off of those fears. Don’t believe the hype.

The color of a mold has no correlation to how dangerous it may be. This is frequently stated by agencies throughout the world including the Centers for Disease Control and Prevention (CDC).

There is no evidence that otherwise healthy individuals have any reason to fear getting sick from general mold growth in buildings, mold inhalation, or any other type of exposure even to the so-called toxic molds. Yes, being around mold may cause minor effects like a stuffy nose or coughing for some, especially those with asthma or mold allergies. Typically, it only seriously affects patients who have underlying health conditions such compromised immune systems who are at risk of systemic fungal infections. But unless you’re in one of those rare categories, you really don’t have much to fear about exposure to any mold species.

That said, we should not have mold growing in our buildings. It is an indication of something wrong and will lead to the degradation of building materials. Regardless of color, all visible mold should be removed from buildings and homes.

Not all black colored mold is that kind of black mold.

I've seen similar things with homes with water damage. One of the shitty things about water damage is that you have to redo a lot of the electrics throughout the house because it can cause corrosion. Corrosion in connectors in things like outlets increases resistance and can become a fire risk.

I'm guessing the whole wall got sustained water damage causing corrosion in the outlet, increasing resistance, which caused it to throw off some heat. The heat dried out the plaster and prevented mold growth.