Had to respond to this, even if it was years ago...
Not that she's the be all and end all of experts, but Dr. Susan Love stated in her book that Black Cohosh was safe with ER+ cancer. It's been studied a fair amount. I was under the care of an integrative oncologist, a regular oncologist, and a doctor of naturopathy, and all of them recommended the cohosh.
However, a caution, be sure if you take the cohosh, it's not a mixed "menopause mix", with soy and other phytoestrogens that ARE NOT a good idea to take if you are ER+.
Just my two cents. It's made a big difference to me on the hot flashes. No improvement in any other area, but at least I don't have the flashes and sweats!