Blossom End Rot is the result of the plant being unable to distribute sufficient calcium to the fruit via its internal system. There can be plenty of calcium available to the roots, the plant just can't distribute it. It's thought to be caused by plant stress.
What causes the stress to the plant? It can be fluctuating temps, strong winds, irregular watering patterns, overwatering, overfertilising - in fact, anything that upsets the plant's internal equilibrium can cause it. Sometimes there's just no knowing. I've had a plant with BER planted right alongside another plant with no BER. Same soil, exactly the same weather and growing conditions.
The further complication is that some varieties - the plums, like Roma and San Marzano - are simply more prone to BER. No one knows why beyond that they seem to be genetically predisposed to it.