Dr. Carolyn Male, who is possibly the world's foremost expert on tomatoes, has written up an extensive article on blossom end rot. Basically, among other things, it says that even though calcium is implicated in blossom end rot, the problem is usually not a deficiency in the soil, and adding extra calcium will probably not help in most circumstances.
Now, you can read that article to see what else she says, but I'll tell you what I have to say about it personally, at the moment: I know from personal experience that adding extra calcium will not always prevent or reduce blossom end rot. I wouldn't even be surprised if it could cause it. It is my suspicion that the following things are the major causes of blossom end rot (in no particular order):
- Planting susceptible varieties (like Martino's Roma, Pomodoro San Marzano, many plum-shaped tomatoes, etc.) Some varieties are genetically predisposed to get it easily, I've read. Some resist it very well, I've also read.
That's interesting what bstpierre says about too much potassium inhibiting calcium uptake. I actually did give my plants a fair amount of potassium (and calcium), this year. Nevertheless, the only ones to suffer much from it were Martino's Roma, Pomodoro San Marzano and Roma x Lemon Boy (or else a mutant yellow Roma). If that's true, low nitrogen could also contribute to blossom end rot. I also wouldn't be surprised if an imbalanced nitrogen to calcium ratio could contribute to it (I didn't give my tomatoes much nitrogen, and I just thought it may have been related).
Edit: My personal hypothesis in 2022 is this: Blossom end rot is caused by an imbalance of any of the following nutrients (too much or too little of any one of them in relation to the others): nitrogen, calcium, magnesium, and potassium. Note that according to my hypothesis watering and drought probably seem to play a role because of how nitrogen is more available when the soil is wet.