Bloodlines were a now obsolete term used to describe Vampires belonging to a specific family, descended from one of the first and greatest of their kind, and as such strictly mirroring their forefathers. Now any Vampire can follow a specific path regardless of their family line.
The original synopsis was that all vampires belonged to one of five distinct groups:
The Blood Dragons, Vampire Knights who focused purely on skill in martial combat.
The Necrarchs, Vampire Necromancers who focused purely on magic.
The Strigoi, Vampire Ghoul-Kin who focused on brute strength.
The Lahmians, Vampire Spies and Assassins who focused on manipulation, murder and intrigue.
The Von Carsteins, "Regular" Vampires who were generally a hybrid of all four.
This was eventually retconned by Games Workshop, Blood Dragons became part of the new "Blood Knight" archetype, along with adding many other families of Vampires, such as the Von Draks and the Von Dohls. With these families including the original Von Carsteins, having members who could follow any path and focus on specific skills. Becoming Necrarchs, Lahmians etc. regardless of the dynasty they were allied to. This allowed for Von Carstein, Mousillon and other faction armies to field their own Blood Knights, have Necrarch Vampire Lords etc.