It's called blossom end rot due to insufficient calcium and not enough daily watering. You can use certain remedies when they are in flower but someone else can probably tell you if you can stop it when it's fruiting.

Thank you

Its usually erratic watering - without regular and sufficient supplies of water, calcium take up by the fruits is interrupted -its quite rare for the soil to be deficient in calcium.

the ripe fruit above the black bottoms is still edible. So don't throw them all away. at worst use them in cooked dishes.

Nobody wants to eat tomatoes with blossom end rot, blah! I've had BER in the past. For whatever reason, the plants cannot utilize the calcium in the soil. Calcium is vital for plant cell wall structure. Without it, the plant cells collapse and mold & fungus sets in. This year my tomatoes are gorgeous, delicious and way too many. I give them a feeding every two weeks. I use Doctor Earth organic vegetable fertilizer and also Alaska Fish Emulsion. Just a couple glugs to a gallon of water every 2 weeks. I'm not sure which of these provide the 'missing link' but my tomatoes have what they need to reach their fullest potential. I posted a pic in my gallery for you. No BER this year.

I realize you may not have access to the same brands that I do,maybe you do. But surely you can find something close.

You can get it on Amazon.com. Here is the link. https://www.amazon.com/Alaska-Emulsion-Fertilizer-Concentrate-Gallon/dp/B002RH0B26

I have BER every year in my greenhouse, but I grow twice as many outdoors and have never had BER yet. And my watering outside is not consistent at all

A lot has to do with how you start your tomato plants off. When you plant them out at the start, plant them deeply. Throw a few handfuls of Dr. Earth down into the planting hole. Deep down, the moisture & nutrients are more consistent and even. They should be able to withstand a dry spell because deep down water is available. We have underground aquifers here on Long Island, NY. They keep the ground evenly moist regardless of the weather.

Information on this disorder in the link below

https://www.rhs.org.uk/advice/profile?pid=395

I broke of a bit off some white blackboard chalk, crushed it and sprinkled it round the plants. The rot cleared up very quickly and didn't come back. I just reckoned that it low calcium was the cause of the rot I reckoned chalk should work- and it did. Very cheap, no bother and plenty of chalk left for next year...

Nice one Sue, I guess that clears things up nicely. Must get chalk for next year.

but you must water plenty and often or the calcium wont be taken up by the roots.

Must have done it right then!