It is ironic a Southern Democrat is the one to start this trend against a moderate originally from the Northeast Republican.
I would argue that Dukakis started this trend, as he won NY, Hawaii, Rhode Island, and Massachusetts; and he scored 40% in states like Maine, Illinois, Maryland, Vermont, and New Jersey, etc. It might not sound like much, but after Carter and Mondale's blowout losses, these results were uplifting for Democrats.
Dukakis definitely laid the groundwork for Clinton's 1992 realignment.
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Anyways, to address the topic:
You ask, "realistically what would it take to make any break in the Northeastern Blue Wall?"
To answer this question, you might want to look at voter preferences in the Northeast. When Republicans get elected in local or state elections here, they tend to be socially moderate; if they are socially conservative, then they keep their views quiet while campaigning (e.g. Guilani, Bloomberg, Romney before 2012, Pataki).
Therefore, if the Republicans want to compete in the Northeast, they can start there -- decriminalize marijuana, don't emphasize abortion or gay marriage, defend civil rights more vigorously, etc.
Still, it must be noted that, even if they make these changes, a massive realignment must occur before we see legitimate competition in this area. Of the 10 most Democratic states in 2012, 6 were in the Northeast:
Wel no not really RI still went for Carter in 1980. Hawaii only goes Republican if there is a Republican Blowout like in 1972 or 1984. Carter even carrid Hawaii in 1980. Massacusetts-It went for McGovern in 1972 but it did go for Reagan both times in 1980(albeit only by 3800 votes because of Anderson taking 15%) and 1984.
New York-You might have a point. The state was still more Dem than the popular vote nationwide in 1984.
Maryland-Carter carried the state in 1980(Anderson took 8%) but Bush HW did carry the state in 1988 as did Reagan in 1984. The States PVI for 1984/1988 was D+4 so it was still a Dem State.
