Real dark chocolate does have high amounts of flavinoids, mostly epicatechin. So far the research shows real benefit in warding off heart disease. I can't find much in relation to breast cancer. However, flavinoids are always a good thing. The caveat is that not all dark chocolate is created equal. look for at least 75% cocoa content or higher. Look at the total glycemic load. In order to do that subtract the fiber from the carbohydrates and put that number over the protein. A healthy chocolate has about 5 grams of fiber and 5 grams of protein with a total carbohydrate listing of 11 to 17 grams total.
Just because the research isn't there doesn't mean it's not helpful. In general, the more flavinoids one consumes the better. Chocolate is up there with green and white tea, rooiboos has even more than green or white tea. Black tea and coffee have lower amounts.
Flavinoids in order, from highest to lowest:
Green Tea and Real Dark Chocolate
Black Tea and Coffee