BER (ber) Blossom End Rot, is has two competeing causes. 1. irregular watering, 2. an inability of plant to take up adequate amounts of potasium. I put on things rich in calcium and potasium even though I have been told for years, it does not work.
For me, not growing roma tomatoes did more to stop BER in my garden, than any addition did. Pick a strawberry shaped tomato or cows-horn instead (cows-tit or gilbertie), to replace Roma's.
Cool weather when your tomato is still tiny, has a lot to do with BER. A couple good thunderstorms and your first set of fruit can look pretty sad.
Lastly the huge blossom scar is something many tomatoes do on first fruit-set. I might not save seed from those tomatoes, but they eat just fine.