If anyone's noticed, the Blood Ravens have no history. They don't know their Primarch, Legion, or anything. GW has slowly hinted that the Blood Ravens are loyalist Thousand Sons. For Example, in the book Dawn of War Tempest, Arihman calls the Blood Raven 'Lost brother of Arihman'. Also, if you look at blackLibrary.com PDFs, the Blood Ravens are the same color as the Thousand Sons before the Horus Heresy. What do you think?