I wrote this yonks ago, just in case anyone may wish to give it a glance.
"Blossom end rot occurs when there are insufficient calcium levels in developing fruit. This is most common when the first trusses are forming and calcium demand is high.
As water transports calcium around the tomato plant the condition is usually linked to inadequate or irregular watering.
Tomatoes need the equivalent of 2-4cms rainfall every week, though the amount of water given is less important than maintaining consistent water levels. Watering daily, at least, will be necessary in hot conditions.
As little as half-an-hour of water deficiency can cause the condition to develop. Pick off and compost any fruits that have been affected.
Mulching around tomato plants with grass clippings, straw or hay will prevent plants drying out."