Could number of powers, modifiers for these powers and so on not be wholly dependent on bloodline strength and the categories - Tainted, Minor, Major, Great, and True - be something that is used as an inaccurate term in game?
The idea of a nasty surprise - expecting your opponent to be of a Major bloodline and therefore inferior to you - and then finding he/she is the equivalent of True bloodline, that is a cool idea.
We don't even need them for ECL purposes: At strength 15+ you get +1 ECL and so on. They're just categories.
If the term has a game-mechanical effect, I think it should be very clearly defined. That does not exclude it being less clearly defined in the game world. In the setting, those terms are something historians, bloodline scholars and so on throw around. The only way to accurately determine someone's BS is through magic and you need the subject present to do that (unless we're talking powerful scrying magic), so only a small number of people actually have accurate information on anyone's BS.