I'm growing 6 tomatoes in containers of different varieties. Yesterday I
noticed that on one plant, one of the set tomatoes has blossom end rot.

BER has all sorts of possible triggers, including disturbing the roots,
uneven watering, mineral nutrition problems (esp. excess N), etc. There
also seems to be a genetic component to BER.
The best treatment I've found for container plantings is drip watering
and a good mulch. For plants in-ground, most years a good mulch is all
that seems to be required.

See: (mind the wraps!)
http://www.extension.umn.edu/projects/yardandg
arden/AAMG/vegetables/ber-tomatoes.html

http://www.coopext.colostate.edu/TRA/PL
ANTS/index.html#http://www.coopext.colostate.edu/TRA.../blendrot.html

http://www.entoplp.okstate.edu/ddd/diseases/ber.htm