OK.
Grand Admiral Thrawn is like a HUGE genius at naval operations against his enemies. He is nearly unbeatable. He is guarded by fiercely loyal guards that have their own history with the Empire and they regard Vader as their savior from a poison plaguing their planet. So, Thrawn sends a group of these assassins to kidnap Lea (or assassinate, but I think it was kidnap). Before I go on, they have an extremely acute sense of smell where they can even differentiate a person's bloodline based on their smell. Ok, now getting back to the story. In their mission to kidnap Lea, they become aware via her smell that Lea shares a close bloodline with Vader. This throws a wrench in the guards plans as they are fiercely loyal to the Empire because of their loyalty to Vader, not really any other reason that is strong as that one. So, I forget the specificity of the plot at this point but basically Lea proceeds to convince them that the Empire actually poisoned their homeworld and that they should assist the New Republic instead of Thrawn. However convincing her arguments were, they truly and highly respect Lady Vader so the higher ups began to shift toward Lea unbeknownst to the Imperial Remnants. Now for another aside, these Noghri are like the galaxies best assassins. I mean Emperor Palpatine went so far as to wipe out any historical record of their planet so he could have them as his assets in taking out his enemies, etc. So, getting back to the matter at hand, Thrawn wasn't aware of this tidbit about the Noghri: that they would essentially hold higher loyalty to Lady Vader than him. So, Lea convinces the Noghri to help in one of the battles against Thrawn. And right when Thrawn learns that Noghri assisted the New Republic in the battle, his Noghri bodyguard assassinates him. This ends the threat of Thrawn, or at least the original Thrawn.
How could this tie in with answering Rey's bloodline question? Well, if Thrawn and these Noghri bodyguards (or just the Noghri) are in the next movie and they come across Rey, they may be able to sense her familial bloodline back toward the Skywalker family tree. I guess it depends on how sensitive their senses are and how diluted subsequent generations of Skywalker's pheromones or such become.
