Point one: The Thousand Sons and the Blood Ravens have a lot of Psykers and the Libarians in both were in high regard, as the Primarch of the Thousand Sons and the Chapter Master of the Blood Ravens were psykers.
Point two: The Thousand Sons and the Blood Ravens both want to find hidden knowledge such as Ahriman's quest for the Black Libary and the Blood Ravens quest for their origins.
It's possible that the knowledge found on Kronos during the events of Dark Crusade and which was destroyed by the Blood Raven was that they were descended from the Thousand Sons.
These points are highly plausible due to the fact that some of the traitor leigons in the Horus Heresy had some of their Legionaires still loyal to the emperor, if that is the case, then loyalists of the Thousand Sons could've formed the Blood Ravens.
This is just a theory, so I may not be right, but it's still a guess.
Constructive criticism appreciated.