There was a thread awhile ago concerning brown mushy patches on the flower
end of tomatoes (Better Boy and Roma VF). The consensus, as I recall, was that
the plants were not getting enough water. Well, I water 6-7 days a week with
drip irrigation, and although the weather is quite hot and dry, I would guess
that this doesn't constitute underwatering.
Furthermore, we have one other plant about 50 ft away that we water by hand 4-5
days a week that exhibits the same symptoms. Could it be a fungus/virus carried
by pollenating insects (bees)? Lastly, adding to the diagnostic difficulty,
only some plants are affected. No apparent pattern...