It makes me count my blessings, in the handful of years I've had the urban farm I've never seen this before. Blossom rot! While I thought it was caused by moisture (imagining it was the cousin of powdery mildew), the google informed me it has something to do with calcium deficiency, most likely caused by inconsistent (and insufficient) watering. This makes sense as the hose extender broke for a few weeks and I couldn't reach to water this guy. Good news is after I cut these off I've been watering like crazy and there are a lot of large green tomatoes. Only one of the new tomatoes has a small dark spot (smaller than these), the rest are free of the dark spots. It's a volunteer plant, but I'd guess it's progeny of the Brandy boy hybrid as it is vigorous and heavy with fruit. A few weeks ago, I sprouted some seeds from last year's yellow cherry hoping it will do as well this winter as last, but I'm nervous that with inevitable crossbreeding it won't be the same. The plants are about 2 inches so wait 6 months for me to get back to you on that one.