Yesterday I bought a pack of cherry tomatoes and after arriving home I discovered that one of them had developed some impressive black mold (it was bigger than the tomato itself!). Of course, I tossed the offending tomato and the packaging, washed the remaining tomatoes and put them in a clean box. Now I'm hesitating though; are they safe to eat?
Yes - the mold is an indication that the spores have entered that tomato, but do not indicate any problems with others. Mold usually enters fruits like tomato through the stem site or damage to the skin. The bits you see outside the fruit are actually the fruiting bodies of the fungus (equivalent of the bit you eat on a mushroom - the rest is below the soil). These fruiting bodies produce tons of spores. You should use the others fairly quickly before any released spores have a chance to potentially start growing in them.
Edited to add: The general advice would be to discard any fruit that are attached to the main one by the fungal body, wash the others well to remove any potential spores off them, dry well (wetness promotes fungal growth) and use within a short time frame.
The USDA has some good advice here (PDF) - with thanks to SnakeDoc for finding this one.
I once worked in grocery produce.
The skin offers a remarkably effective protective layer. I have opened crates of tomatoes where one has completely turned to mush, while every other tomato in the crate is pristine. Same for apples and pears and every other kind of fruit, really. Wash well and the rest are fine. This is standard practice commercially.
As an aside, as other several popular questions on this stack go into, most of the molds growing on fruit are harmless to humans anyway. You could probably just eat that rotten tomato, especially if you cooked it. Though I wouldn't really advise it.
This is a topic that already has a lot of input from Quora: When one tomato gets moldy in the package, is it still safe to rinse off and eat the remaining tomatoes? Most of the 21 answers there says the rest of the tomatoes will be okay to eat, and I agree.
Also, as one answer puts it: "I see no problem what so ever, with a small amount of mold than can be trimmed. However, I go by smell. If there is an off odor. Toss it." But of course, since yours is a cherry tomato, I doubt you will be able to trim the mold off; just throw that moldy one out, and don't forget to thoroughly rinse the rest of the cherry tomatoes out.