There has definitely been a couple of other hints:
In the Black Library book Dawn of War: Ascension, it is revealed that the ancestors of the Blood Ravens built a recruiting world on top of an ancient Necron outpost during the Horus Heresy, with the help of the Eldar. The two races worked in unison, using Eldar magic and psychic wards to lock the Necron force lying beneath the sands in psychic stasis. The Heresy-era Space Marines were then tasked with protecting this outpost by the Eldar, to seal in the Necron threat, but they mysteriously abandoned it. It was left unrecorded in Imperial logs until the Blood Ravens rediscovered it. The Eldar mistook the Blood Ravens for being one and the same as the Heresy-era Space Marines, as they too wore similar red Power Armour.
The physic lock the Marines placed on the Necrons shows that the Marines of that time could not have been Blood Angels, and the Thousand Sons already share a bond with the Blood Ravens because both groups of Space Marines were defined by the possession of unusually strong psychic powers.
In the novel Battle for the Abyss, a Thousand Sons Chaos Marine shouts "Knowledge is Power!" as a battle cry, suggesting a connection to the Blood Ravens.
In the novel A Thousand Sons by Graham McNeill a vision is seen by the Thousand Sons Librarian Ahriman describing a future image of blood and a raven, and a remembrancer psyker attached to the Thousand Sons has a vision of "...The Ravens, I see them too! The lost sons and a Raven of Blood. They cry out for salvation and knowledge, but it is denied!" (p.430) This may imply that the Blood Ravens are descended from the Thousand Sons' gene-seed. If the Thousand Sons are not the origin of the Blood Ravens' geneseed, then Games Workshop is clearly throwing out a multitude of red herrings.
Avitus mentions that Davian Thule found some recordings relating to the founding of the Chapter on Kronus. He was the only one to have seen them, and destroyed them immediately after. Tarkus notes this as being a turning point marking a significant change in the Captain's personality, now grimmer and more distant than he once was. One could guess that whatever recording Thule found established the Blood Ravens as being related to one of the Traitor Legions. Later in the game, Avitus mentions the fact that the Blood Ravens have forgotten the name of their Primarch and this could not have been an accident, it had to be done on purpose to hide "a terrible truth".
http://warhammer40k.wikia.com/wiki/B...nd_Sons_Legion