Massachusetts hasn't had county governments since 1997-2000. There hasn't been annexation since 1912. So no multi-county municipalities. But we do have the whole unofficial areas with official zip codes thing.
Chestnut Hill (where BC is) isn't a real municipality; it's a "village" partially in Boston, partially in Newton, and partially in Brookline - but it has its own zip code, and you better believe Boston College addresses say "Chestnut Hill" and not Boston, Newton, or Brookline.
Basically every neighborhood in Boston does this too. You don't send a letter to someone in Upham's Corner to a Boston address; it's Dorchester, MA 02125. I have an aunt who used to live in the town of Rowley on the North Shore, but her postal address was for Byfield, MA 01922 (an unofficial village within Rowley).
There are probably over 100 "villages" in MA with their own zips that have no official standing whatsoever, but people still treat them like separate towns. It doesn't help that the Census sometimes treats these villages as separate designated census places. But I guess that's the entire point of a "
Census designated place".