At least, that's how the requirements of drama would go. It would be too big a plot complication, not to mention too much of a tech contradiction, if the Borg could somehow block a swinging fist.
Well the requirements of drama will make the Borg as tough or vulnerable as they need to be to serve the story that week, which is why they appear weaker on Voyager, as the little Fed ship has to get away every week.
You have a point about identification, but surely the software controlling the shields is advanced enough to recognise a blade travelling quickly enough to wound? Even software of the now could probably do that.