Thus Alliegiance is the standard, so parentage citizenship is the deciding factor.
No, not at all. What Blackstone is describing here is
jus solis, not
jus sanguinis. Meaning that your allegiance comes about by virtue of your having been born on the soil, or territory of the kingdom. It has absolutely nothing to do with parentage, which is
jus sanguinis.
Blackstone makes this quite clear. Read it again:
The first and most obvious division of the people is into aliens and natural-born subjects. Natural-born subjects are such as are born within the dominions of the crown of England, that is, within the ligeance, or as it is generally called, the allegiance of the king; and aliens, such as are born out of it.
Blackstone is clearly saying here that simply being born within the territory of the kingdom is what causes you to be born within the allegiance of the king, and that this form of citizenship is what is considered to be "natural-born." Again, it has absolutely nothing to do with parentage.