Thanks for your replies. I have lots of thoughts in response, but the important one is to do with Matthew's wording. "To this day" suggests that, although some time had elapsed, it was known by occupants of the city at the time of writing.
For example, if the writer was writing after AD70, a long way from the city, a more appropriate way to put it would be simply, "that field became known as the field of blood" and leave it at that. 30 years ago my college friends used to meet in a pub called "The Battle of Trafalgar". It became known as "The Traff". I probably wouldn't make a point of stating that it is still known as that, if I were to write about it (especially if London was in ruins). Matthew's point is more to do with the prophecy being fulfilled. Why would he point out that it was still known as the field of blood, when it was derelict and nobody lived there?
Again, who is he referring to if it's post-70? Not the residents of Jerusalem - only the Roman garrison remained. His Christian circle of friends, or his Jewish acquaintances? To know something by a name implies knowing the thing itself. Nobody would be talking about it enough to merit a mention, if they did remember it. It wasn't an important detail unless the time of writing the account is pre-70.