I have had the same several times - blackish/brown bottoms?? Apparently it is inconsistent watering. I find this difficult to accept as I am as regular as clockwork - but maybe it's watering too much? I destroyed the affected ones and the rest are fine.

I think its when its very hot they are unable to take up enough water--- something also about the nutrient imbalance? -- in a morning or when its shady ( not last thing at night ) I spray them all over the leaves and although i've had some ' black bottom' ( really called blossom end rot) its been quite good this year