I think this is either a watering issue or nutrient issue.
I didn't have any real issues with blossom end rot except for the one container plant.
These were all 'big boy' plants, started from seed. Many of the first tomatoes were good sized, but from mid-production up until now (I still have some green ones left to ripen on the vine) the majority of the tomatoes have been no larger than a tennis ball...yet taste fine and are juicy.
Not enough N, P, or K added once they started blooming? This was the first time growing tomatoes, and we had to prep the ground in March of this year.