The Blackstone definition is for “subject,” of course, since that was the English common law equivalent of “citizen.” But it is basically jus solis, determination by location of birth, not by parentage.
Blackstone: 1765
http://press-pubs.uchicago.edu/founders/documents/a1_8_4_citizenships1.html
There have been a number of Supreme Court decision with peripheral bearing on the issue, but none are definitive. In particular, the 14th Amendment may have altered the meaning of the terms used in the original constitution.
Personally I wish the Court would take such a case and just define the term. That is exactly what the Court should do, clarify ambiguous points in what the Constitution really means.
I don’t accept that the definition of “B” (subject) is dispositive in defining “A” (natural born citizen).
The Supreme Court decision that I believe you are referring to is Minor v. Happersett, which to my knowledge is the only one to have defined “natural born citizen”. If this is part of the holding, as has been asserted, and not dictum, then it would be dispositive in construing the term “natural born citizen”.
I don’t see how the 14th amendment amended the definition of “natural born citizen”, since the term “natural born citizen” was not used in the 14th amendment. I do note that the Citizenship Act of 1790 extended the status of “natural born citizen” to people both of whose parents were US citizens, even if those people were born at sea or outside of the US. That law was superceded by the Naturalization Act of 1795, which did not use the term “natural born citizen”. However, one could use the 1790 act to argue that congress, in promulgating that law, understood that a “natural born citizen” had to have two parents who were citizens (unless born before the US was organized).