I can’t be elected President even if I satisfy everyone that I have no loyalty to the USSR, where I lived until I was seven, or to the nation of the Ukraine, which didn’t exist as a country when I was born there. Conversely, someone born in the U.S. (and thus the American equivalent of Blackstone’s “natural-born subject”) is a natural-born citizen regardless of whether we think there’s reason to doubt their undivided loyalty—say, the parents’ foreign citizenship at the time of birth (a pretty weak reason for such doubts, I think), the citizenship of the candidate’s spouse or children, blood relation between the candidate and some foreign potentate, or whatever else.
The voters can certainly consider any claims of dual loyalty. But the minimum criteria set forth by the Constitution are solely that the person be “a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution,” “have attained to the Age of thirty five Years,” and have “been fourteen Years a Resident within the United States.”
I wouldn’t vote for Kamala Harris for Vice-President (or for President); but she is indubitably constitutionally eligible to the office.