I copied your post into the comments of this page, to keep track of them. I feel bad that I didn’t take the time to dig into Blackstone on the matter for the article!
http://investigatingobama.blogspot.com/2008/12/donofrio-dual-citizenship-natural-born.html
Feel free to add any further comments.
After further analysis of the comments attributed to Blackstone, one can see that he is observing the two criteria for being a "natural born subject," of 1. birth within the nation and, 2. birth citizenship passed from one's father's citizenship, within English law.
However, as I see it, he does not attempt to induce a broad principle in the event of one meeting both standards. It would seem that the scribes of English law saw these two principles of natural law and deduced each to be allowable particulary for English law.
However, Blackstone did observe the problem where these two criteria of citizenship conflict (as did Jesus Christ) that one "cannot owe two such allegiances, or serve two masters, at once."
It would be interesting, whether meritorious or not to American Constitutional law, to know how English law treated allegiance and "natural born" allegiance when a child of a citizen/subject of another nation was born in the lands of the English crown.