What gives, you do everything you can for your tomato's and this is how they repay you.
Blossom End Rot(sounds like a good name for a punk rock band) has got something to do with not enough calcium being available when setting fruit.
Calcium deficiency in toms can be caused by to acid a soil or in my case poor watering as will be revealed.
I planted my Roma Tomato's in soil following a crop of peas for which the soil was limed,
therefore there should be plenty of calcium and a corrected soil acidity.
The soil in this particular part of the garden also maintains a fairly good moisture content so I thought I had all my bases covered, but, you hear me say, but, and I new this, but, and I thought I had it covered, I used that fine sugarcane mulch that comes compressed in plastic bags to mulch around my toms.
How is this a problem I here you ask, well let me tell you.
It has to be the most hydrophobic substance on the planet. I can stand there with 2 watering cans, that's 18l of water, and it manages to shed all the water away from where you mulch with it.
So the cause of my Blossom end rot is poor and infrequent watering because the bloody sugarcane mulch wont let any water through. Which in the end makes it my fault because I already new this but persisted in using the stuff anyway.
All is not lost though, I have scratched the mulch away and am now getting water to the base of the plants and they are now setting a good crop of fruit.
I should have done this a few weeks ago but I'll also be adding a side dressing a potash to help with with flowering and setting fruit.
By coincidence I see Aussie Organic Gardening has a post up today about this very topic and you might get more insight into blossom end rot from there.