Here in PA, I sometimes get BER with heirloom plum tomatoes as a result of underwatering. Technically, it IS a calcium deficiency, but if you are growing in soil or with well water (or both), there is usually enough calcium present for the plant.
Calcium does not mobilize well in tomatoes, despite their high demand for calcium. You can folar feed all you want, but calcium is not readily uptaken through foliar feeds (several universities have shown this in independent studies - my education on this was at UMass).
"High" levels of calcium for tomatoes is a relative term. Healthy tissue samples only have ~14 ~ppm equivalence Ca in them.
This is why you see a lot of hydro nutrient with up to 200 mg/L - the plant can't possible use that much, but they have to overcompensate for the fact that Ca doesn't mobilize well.
BER can happen because of both under and overwatering. The leading cause is uneven watering - going from really wet to really dry and back again. Folks tend to overwater tomatoes, but keeping them above 25% field moisture at the root zone under all circumstances really helps.
BER tends to happen more with potted tomatoes or in grow bags much more than those in the ground since moisture fluctuations happen faster and more frequently.
Best thing to do is to at this point is to pull all the 'maters with BER, let the plants concentrate on the fruits that haven't gone bad and make sure field moisture in the root zone don't fluctuate too much. I use "on ground" hydro outdoors with drip tape and found that they tomatoes have a WAY different moisture requirement than my rows of superhots.
Don't get caught up with "it needs CalMag" or "I'll give it Epsom salts (which are Mg, not Ca, and is uptaken through the same channels)". 9 times out of 10 BER is a directly result of available water at the root zone.
for reference, plum tomatoes are most suseptible to BER, followed by big heirlooms. Cherry tomatoes are least affected by BER, and my favorite, Oaxacan cherries, never get it - you can overwater 'em, underwater 'em, they just don't care and continue to push out fruit for months.
Good luck and don't get dishearteartened - BER sucks but you can correct it in a matter of a few days by watching your watering carefully.