What Causes Black Spot or Blossom Rot on Tomatoes
There is also blossom rot on my green peppers and zuchinni.
Certified GKH Gardening Expert
This is normally due to erractic watering and a calcium deficiency. You can simply use a high calcium fertilizer, or add some limestone or calcium carbonate to the soil around plants. Here is more information: https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-blossom-rot.htm
Over zealous watering by my husband (fear of losing his babies) caused blossom end rot but the good news is that when we watered more deeply and less frequently, the subsequent tomatoes formed beautifully and without ugly bottoms. We also added lime to half the plants and none to the others and the results are the same so I believe it was the watering. Good luck,