Pelargonium leaves issue.

Has there been a sudden change to environment? Did you recently move it? It could be reacting to some sudden change - going through a shock period, which it will recover from. Open it up to give as much air circulation as possible. They don't like a lot of water. Let it dry out between waterings. Avoid getting the foliage wet.

that is typical of a very cold night and perhaps it was just a little more exposed that the others.

That's interesting. It's the only pelargonium that I've kept outside in recent weeks. We haven't had any frost, although we've had a few nights where the temp dropped to 5° C. But I've found pelargoniums can handle that temp without a problem. The leaf blackening only became noticeable in the last week, and the soil had become dry, but no wilting.

That is the plant's response to chilly nights. It thinks that it may frost soon, so it produces a crimson pigment in its leaves that acts as a natural antifreeze. Different varieties vary in their responses to cold.

I was about to say exactly what Tug said. This is response to cold. Your plant is feeling chilly. My tomatoes do the same. They should be OK once it warms up.