It comes up all the time on various boards, but it's still fun to debate. I don't think there is any one definition that works, and I think it's a combination of a number of factors, many dependent on where you are. Setting the line simply at the point where it's hard to get into port before the weather hits doesn't really work, IMHO. If you're halfway between Fisher's Island and Block Island, you have no chance of getting to port before a fast moving system comes upon you (at least in a sailboat). Same is true between Block and Martha's Vineyard. And even though those stretches are exposed to the ocean from the south and east (basically you sail in the lee of London), I wouldn't really call them "bluewater." Though maybe I should reconsider that, as it would mean I would have many many more "bluewater" miles under my keel. Hmmm?
Oddly enough, I think water depth also plays a little part in this analysis. Not because it matters intrinsically whether the water is 50 or 5,000 feet deep, but once you are "off the shelf," you really are out there. The wave pattern also is much different. Likewise, at some points on the Bahama bank you can't get to port too easily and you have miles and miles of sea room, but can you possibly claim to be in bluewater when your depth sounder reads 12 feet and you can just drop an anchor at any point and ride out whatever's hitting you?
I actually think some (not all) stretches off the NJ coast have several attributes that make them feel like bluewater, if they are not in fact so. The water is very deep, there are no harbors of refuge that are reachable as a practical matter, and in anything with an easterly component you feel the wind and sea just like you would if you were 100 miles to the east. I appreciate that if you're only a few miles offshore you're hard-pressed to call that "bluewater" (particularly off the Jersey coast where it's mostly brown water; just a joke, keep your swords sheathed), but that stretch most definitely has some attributes that are akin to bluewater sailing.
All that blubbering said, if you had to pick one attribute that "defined" bluwater sailing, I would say the inability to get to port without an overnight passage puts you in bluewater territory, with the caveat that there are places on the planet that would constitute exceptions, going both ways.