I'm not sure if this is the cause. But when I don't mulch my tomatoes -this happens. Something about rain hitting soil and bouncing back on the fruit.

I've noticed this happens when we have a dry spell followed by rain. I agree that the mulch helps considerably with this. Maybe it is a soil borne organism like Baumer suggests, I never thought of that...I thought the mulch was helping conserve moisture.

Blossom end rot is caused by a deficency of calcium in the soil... Here's a link that will tell you more than you ever wanted to know about it. ;)

Diann
IA Z5a

Here is a link that might be useful: Blossom end rot

It can also be caused by uneven watering of the plant, and is sometimes a problem in container grown plants, but it is more common on the first tomatoes the plant produces, the later ones should be ok unless there is a real calcium deficiency or water problem.

The CAUSE of Blossom End Rot is calcium deficiency....but that deficiency is most often caused by insufficient water or fluctuations in soil moisture. Potted tomatoes are very suceptable.
Linda C