I drink darjeeling tea all day, every day. Now and then, I have a cup of coffee (to which I add chocolate and milk). They are my main source of fluids (although I also drink .75 to 1.5 liters of filtered tap water a day).
It is a myth that caffeine causes water depletion/dehydration:
http://www.mayoclinic.com/health/caf...drinks/AN01661
http://www.nytimes.com/2008/03/04/he...on/04real.html
Apart from that, I have not found any convincing evidence that tea and coffee contribute to breast cancer, and most of the evidence says just the opposite. The only caveat is that it seems best to avoid green tea during treatment because some of its constituents have been shown to block the effects of at least one cancer drug (and possibly others).
The problem with black tea is that it may contribute to anemia by blocking non-heme iron in the diet (which would've been a good reason for me to stop drinking it while receiving AC, but I'm an addict).
Finally, my grandmother drank black tea (mostly orange pekoe) several times a day, every day for her entire life. Although she was diagnosed with colon cancer when she was in her mid-to-late 70s, she lived to be 101. And the only reason she died then is that the colon cancer came back and was not caught in time. She was in otherwise perfect physical health (although her mind was pretty well paste at that point--she had non-Alzheimer's dementia, probably related to thyroid malfunction/depression).
(I've just re-read my post, and I see that I'm vehemently not-humorous. I apologize, but as I said above, I'm an addict, and I take any threat to my drug of choice very seriously. Make that over-seriously. Sheesh.)