A kind of odd question, perhaps, but I think my knowledge of vampire lore may be out of date. I used to play Undead when they were one army (4th edition) and then VC when they first split (5th edition). The 5th edition book introduced bloodlines, which I think was a first at the time, but didn't to my knowledge include Strigoi so obviously there have been some developments since then.
Anyway, in the 5th edition army book, Thralls are basically the lowest level of vampire: you have Lords, Counts, and Thralls. However, I was thinking of running a Vampire Blood Bowl team and there there is a distinction between Vampires and Thralls, where the latter seem to be basically human servants (and sometimes prey). Neither source, so far as I'm aware, is particularly explicit about what Thralls are.
Is this just a case where BB fluff differs from that of WFB? Or has what a Thrall is changed over time?