There are two possible answers: 1) Your fertilizer regime is providing too much potassium and 2) Your tomato cultivar is a San Marzano or relative.
Given that you're supplying adequate moisture, I recommend completely stopping the use of fertilizers at this point. Too much potassium can restrict the uptake of calcium. From that link:
...according to Dr. Luke Gatiboni, an associate professor in Crop and
Soil Science at N.C. State University, “it is well known that nutrient
interactions occur in soils, meaning that the amount of one nutrient
such as potassium if in excess can negatively impact root uptake of
other nutrients such as calcium and magnesium; research has validated
this. However, supplying calcium, magnesium and potassium in ideal
ratios that exceed plant requirements may indeed create nutrient
imbalances and deficiencies.”
Let the current fertilizer situation at least partially rectify itself via leaching. This should correct the problem. Here is more information US Michigan State University about preventing BER. If you haven't already done so, I recommend that you have a soil test done if the problem doesn't resolve via leaching.
I mentioned San Marzano tomatoes earlier in this answer. Like many of the paste tomatoes, these suffer from BER but IMO but get it worse than any other paste tomato. I've grown them within feet of other varieties that never showed BER at all while losing half of my San Marzano harvest to BER. I've stopped growing them for that reason.
