Blossom end rot - usually from over watering.
BER is caused by a calcium deficiency, usually induced by fluctuations in a plant’s water supply. Because calcium is not a “mobile” element in plants, even brief changes in the water supply can cause BER.
Droughty soil or damage to the roots from excessive or improper cultivation (severe root pruning) can restrict water intake, thus preventing plants from getting the calcium they need. Plants also can develop calcium deficiency and BER if they are growing in highly acidic soil or are getting too much water from heavy rain, overirrigation, or high relative humidity.
This seems much more believable than 'tomacco' which is the first place my mind went.
Bart: Bleh! Tastes like cigarette butts!
Marge takes it away. Marge and Lisa look it over.
Bart: Gimmee! I want more!
Love me some Simpsons' references
This is the correct answer
To add further, most people with blossom end rot are watering incorrectly, ie a little every day, when you should water a lot but less often (about every 3 days, depending on your climate). Mulching will also help you manage better, and testing the humidity of the soil before watering goes a long way.
Remember, tomatoes are plants from warm regions, they can handle a little drought but they do much worse if they're over watered. Watering deeply and less often also encourages them to develop deeper, stronger roots.
Yep. Mine did that like 15 years ago. It was freaky.
blossom end rot
Eggshells take too long to dissolve, even when powdered, to fix this issue quickly. Grab some bonide rot stop or fertilome yield booster which are liquid formulas you can dilute and add to stop end rot. I had this issue with certain varieties of tomato when grown in pots, and I discovered that watering once a day was not enough, and the tomatoes depleted all the nutrition in the soil pretty quickly. I mulch and then water twice a day when it gets above 90f, and add yield booster every few weeks.
OP, what is your watering schedule, and do you mulch? This will probably be the larger cause of your blossom end rot. Yes, you can try and save as many untouched tomatoes immediately by adding a tomato specific fertiliser or calcium, but your real issue is probably watering.
