"I've always wondered just WHAT THE HELL IS THAT BAAHSTON ACCENT ABOUT, ANYWAY?"
An accent that nobody, NOBODY outside of natives can do properly in movies. It's also disappearing quite rapidly, but it's very old. If you read any of Paul Revere's original writing you can "hear" the accent even way back then, because Revere spelled words like he spoke them.
A true Boston accent is heard as far west as I-495, as far north as Salem, NH, and as far south as, say, New Bedfid. The crap that comes out of Ted Kennedy's mouth is not a Boston accent. Kennedy sounds like a picklesmootcher pretending to be from Harvard pretending to be Thurston Howell III.