Yes you can get blackmouth from the beach in the winter. Not as easy as it used to be in my opinion, but if you put in the time you'll be rewarded.
I can think of a couple of beaches in MA 9 that have yielded BM for me.
I used to catch a fair number of hatchery fish during the winter in MA 13. This was quite a few years ago during the heyday of south sound blackmouth fishing. One beach in particular I fished almost aways produced fish.
The largest I saw caught was a 12 lber by a fellow flyfisher. Just a beautiful winter beach fish. Most of the fish I seemed to encounter were in the 4-7 lb range which put a nice bend in your rod and tasted great as well.