Full disclosure: I live between Davis and Ball, don't have a car, take the T every day, and do just about all of my grocery shopping on foot. This means that over the last 3.5 years I've probably carried food home from the "Porter Shaws" close to 500 times.
The way I see it, bfresh will rely about 100% on Davis T users who buy groceries on foot. People who don't T through Davis probably won't shop there, and people who do T through Davis but drive to the grocery store probably won't shop there either. This leaves a core group of customers, but not a huge one. I wonder if that group will be large enough to keep the store in business.
I also wonder what bfresh, if it's successful, will do to McKinnon's. On the one hand, McKinnon's now has a competitor directly across the street. On the other hand, I know there have been times when I've purchased meat at the "Porter Shaws" instead of at McKinnon's only because I also had other groceries to buy and didn't want to make trips to two squares. I can imagine shoppers now going to McKinnons+bfresh who previously would have gone somewhere bigger.