Possible Connection to the two Lost Primarchs
In the Dawn of War series of computer games and books it is unknown who the founder of the Blood Ravens Chapter is. The two lost Primarchs were numbers II and XI; the only information Games Workshop has ever released that directly addresses them can be found in False Gods and The Lightning Tower (by Graham McNeill and Dan Abnett, respectively). In the Black Library Horus Heresy novel series, Horus, in a Chaos-induced dream sequence, apparently goes back in time and sees the cracked incubation capsule of Primarch XI before the Primarchs are scattered to the Warp (the effect this has on the Primarch is inconclusive). Before that happens, he puts his hand on XI's capsule and feels "the untapped glories that might have lain ahead for what grew within, but knowing that they would never come to pass". This probably rules out Primarch XI as the Blood Raven's founding Primach. There is little to no information on what happened to Primarch II.
There is a moment in the audio book of the The Lightning Tower (Dan Abnett). As Primarch Rogal Dorn is constructing the defenses of the Imperial Palace (in preparation for the incoming attack by the Traitor Marines), he comes across a corridor showing statues of all twenty Primarchs, including the two missing ones. The audio book states that "an accident befell them that may somehow be a precursor of what happened to Horus". In the novel Mechanicum it is said that the two Primarchs are "lost to us forever."
Since the Blood Ravens do not know from where their geneseed came, they refer to their Primarch as the "Unknown Primarch". Based on Davian Thule's speech if North Vandea is invaded, the Blood Ravens feel a spiritual connection to their Primarch, even if they do not know his identity.
During Dawn of War 2, banter between Blood Ravens Thaddeus, Cyrus, Tarkus and 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".