I don't think that reapplying after rejecting an acceptance will necessarily put you on a blacklist, as long as you have legitimate and sensible reasons. I know of a girl who turned down an acceptance from NYMC, then reapplied and ended up faring much better in her application the following year. Mind you, she was declining just one acceptance which was from a third-tier school.
The thing you do have to remember is that the whole process is a crapshoot and things could conceivably go wrong next year and you might find yourself with zero acceptance. If you got multiple acceptances this year, you should consider yourself fortunate and you should not assume that you'll get the same results next year. As others have said on this thread, hold onto those acceptances. Ask those schools for deferral. Some schools will grant deferrals more readily than others.
If you cannot get deferral, then I think you will have to choose between making the sacrifices to start med school this fall, or to possibly setting yourself back a long ways in your goal of becoming a physician. If you were like me and many other applicants, before securing your first-ever acceptance, you probably had the mindset that you'd do anything to get into any one med school. Now that you've gotten your first acceptance and then the second and the third, it's natural to start thinking of better options, to start seeking flexibilities and start demanding more ideal situations. But you need to be very careful not to lose sight of how precious a medical school acceptance is, how long a road it was for you to get to this point, and how a post-acceptance change in mindset might be affecting your thinking.
