Blackstone is telling you right then and there that there are SIGNIFICANT differences between the privileges of ACTUAL natural born subjects and the "generally speaking natural born subjects" on which he opined.
You're calling me stupid when YOU can't understand what Blackstone actually wrote? When you can't tell the difference between fact and an opinion qualified by a disclaimer? Look in the mirror when you say that, man.
Blackstone included a disclaimer because not all children of aliens born in the UK were NBS.
And you cannot understand law by reading sentence fragments. Sometimes you need to read paragraphs, or even pages. But in your case, we may need to provide animation...or really large crayons.