If your pots have drainage, and aren't left standing in water in any outer tray, water more and more often. Blossom end rot is caused by the fruit's inability to access calcium because its too dry at the roots. Correct the watering and it should solve the problem - comfrey does contain calcium, so it won't be because the calcium isn't there, it's just the plants can't access it because of insufficient water. It's next to impossible to overwater anyway if your pots can drain, and once fruit is present, stepping up watering is a good idea.

Pick off any affected fruits currently present on the plants.

I agree with Bamboo, I would guess that watering is the problem. Although you water twice an day, I would say that 10'' pots would be a bit on the small side and at certain times of the day larger plants will be short of moisture between watering times.