In my opinion, your decision should be based upon presenting an image which enhances, rather than detracts from your candidacy. At my interviews, nearly all of the male and female candidates wore suits of some sort and yes, the most common color is black. I did see some amazing choices of attire, including a woman in jeans and a sweatshirt, and all I could think, as a fellow interviewee, was "What were you thinking???" I know I would not want adcom personnel to wonder that about me.
In general, you should strive for conservative professionalism in order to respect your audience and convey that you are serious about entering a conservative profession. You should appear neat, polished and comfortable. It is fine to express a bit of your personality through your choice of blouse or accessories, but nothing too bold, revealing or otherwise inappropriate.
Med school interviews just are not the time to make fashion or political statements. You should wear clean, conservative shoes and if you carry a bag or case, it should be professional and in good condition (backbacks, canvas totes, etc just do not look nice). Choose your nicest, comfortable shoes; if your feet are happy, you will be too. If you feel good about yourself and your appearance, this will give you the confidence to help you excel during your interview.
Best of luck to you!