BER is something you often get with tomatoes (peppers and other things, too). It's attributed to many things, including that some varieties are just worst than others, a lack of calcium, and the things noted already, plus many more. Some people say BER sprays work, others say they do not. I did a lot of research about it last year, but not sure how much I posted in this forum about it. You might want to do a search and see if I found anything helpful.
I have found I have it a LOT more in Earthboxes than I did when I gardened in soil, which leads me to believe its more of a nutrient issue. My EBs are never without water whereas I often didn't get to my very large garden as frequently as I should have. Yet, in my highly organic soil (in ground gardening) that I never used conventional ferts on, I never had problems. I honestly don't think I threw away more than 20 tomatoes in 10 years of in ground gardening and I threw away 20 tomatoes on one plant alone last year in an EB. I've also heard anecdotal evidence that people who use EBs have less the second year if using the same soil. Hmmmm.
This year I made sure I worked lime into my EB soil a few weeks prior to planting. I also made sure I used a fertilizer with calcium and magnesium. So far, no BER. But, I got things in a bit late and I don't have that many large enough to show it. I still have at least 10 on various plants and none have it.
Last year, I put the lime in the day I planted. It took a long time for the BER to go away. On the EB forum, one of the people swears by adding a one time shock treatment of lime and a weekly or biweekly "snack" of lime.
Here's the link to a discussion about it. I've always shied away from this because you can make things much worse adding a lot of one nutrient, then throw things off, and chase your tail. BUT - lots of people swear by this. I have not tried it, so have no first hand knowledge of it. Another experience gardener says that the only solution is waiting it out and these people are using lime at about the time the problem is self correcting, so they think it is helping when it isn't. Again, I have no idea which is correct, but do think calcium is involved. I just don't understand how the plant could uptake that much calcium and get it into tomatoes already forming and stop the process with one treatment. But, until someone does some tests, I'll be the first to admit my hypothesis isn't any better than any other one!
And, given that many fertilizers to not have added calcium, adding it might be necessary if you're using one that doesn't have any, though not sure how quickly it will resolve the problem.
All of the above is most appropriate to EBs/container gardening as there are no nutrients naturally occurring in potting mixes, unlike in the ground. However, depending on the ground, a person could be missing important nutrients and might also need to add something. For in ground, a soil test from a reputable source (rather than the kits that won't really tell you much), would be the best starting place.