A blood test for the force was full on retarded. The empire would have everyone forcibly tested if this was actually a thing. The thing with space magic, if you get me to accept it is there, do not over explain it. Let it ride.
In the novel, Tarkin, the Empire did have almost everyone tested. They tested anyone working in the government, armed forces, and big companies. Likely there were always some folks that didn't show up on the census even in the Core Worlds that slipped by and testing was probably very spotty the further from the Core you got.
They never mentioned if Leia was tested or if results for her tests were faked, but that's always possible.
Yeah, but wasn't that written long after the prequels...in other words a retcon of sorts? I could accept they would miss people, but it was not a thing in the trilogy...George Lucas felt the need to over explain his space magic...and authors try to make it make sense after the fact. What I do not understand though, with the originals, is how did Vader not sense Leia to be force sensitive when he was in such close proximity to her.
I can imagine that Leia was not aware of her force potential, and since it was only potential, didn't manifest in a way that Vader would necessarily pick up on.
Luke only pinged on Vader's senses during the trench run, and only when Luke was able to avoid getting shot down as easily as his buddies.
Also worth noting is that Force potential by itself doesn’t mean much beyond what looks like luck/quick reflexes unless you’re trained and a 19 year old is so far beyond the age when a Jedi would start their training that they probably aren’t seen as a particular threat. Being strong in The Force might explain her proficiency in politics at such a young age, but that’s the extent of the threat she posed.
The other thing is that, based on Leia’s comments, she’d already met Vader before the Tantive IV’s capture. If so, Vader would have noted her sensitivity and investigated it then. Being a senator’s daughter/senator probably insulated her from being taken to be an Inquisitor and without training she would not be deemed a particular threat (Ezra only became viewed as a threat once he was observed to be training with an apparent Jedi).
In other words, by the time of Ep 4 Vader would have no need to remark on her strength in the Force.