I've read that it might be a magnesium thing.
It can be. If that is the case, occurrence of BER in the first fruit tells you its about a month late with the Epsom Salts.
{its in the pharmacy with the foot soaking stuff} It says how much to apply to tomato seedlings on the box. Next year, you might remember to put a tbsp or so in the hole when you plant seedling. You can, and should make corrections now... but its not as fast acting as ammonia nitrogen.
As soon as I noticed it, I put a cup of epsom salt in my batch of compost tea, then sprayed that on the soil all around the pepers and tomatos.
ANOTHER prime culprit is inconsistent watering. Bone Dry, too wet, bone dry.. is a good way to get it.
ALSO Getting the foilage wet, say if your watering regimin consists of blasting the garden with a rainbird sprinkle. Apparently, drops of water get in the blossom and turn into a rotten spot. Thats why Drip Irrigation is advocated for peppers and tomato's.
I am rigged up with drip, and still get it on some of the first fruits, and occasionally a few throughout the season. Next pring I will just broadcast a whole box of epsom salt everywhere, and give it a chance to work into the soil.