Best way to combat is even watering.... It's not so much a calcium deficiency as it is maldistribution of calcium through the fruit. The blossom end does't get enough calcium (usually due to poor watering strategy) and turns black.
Funny thing is, I haven't watered mine in a couple months, but no BER, I guess they have really deep roots. The roma / paste type tomatoes are more prone to BER, and I noticed it on a few of mine earlier in the season but not now.
It could be a deficiency, but you won't know unless you get your soil tested.