It's most often not a calcium deficiency, but the plant failing to evenly distribute calcium through the fruit and to the blossom end. With the deficiency in the blossom end, that is the side that rots and that's why it's called blossom end rot. They always say the best remedy is even watering. However you might want to have your soil tested just in case there is a deficiency, but it's not that likely. Also, some varieties are more prone to BER than others, especially romas. You will see a lot of BER on roma and other plum-type tomatoes, some on beefsteak and slicers, and next to none on cherry tomatoes.