240-386.M41 (40,240-40,386 AD): The events of the EISENHORN trilogy take place. The RAVENOR trilogy takes place within a century of the end of this series.

741.M41: First official contact between the Imperium and the Tyranids when Hive Fleet Behemoth invades the Tyran system.

755.M41: The Sabbath Worlds Crusade begins, marking the rise to fame of Comissar-Colonel Gaunt. The GAUNT's GHOST novels take place over the next 20 years or so.

801.M41: Hive Fleet Naga invades the Galaxy and is destroyed by an allied force of three Eldar craftworlds.

899.M41: Hive Fleet Gorgon is destroyed by an alliance of convenience between the Tau and Imperial Guard forces.

919.M41: Ciaphas Cain joins the Valhallan 12th Field Artillery. The CIAPHAS CAIN series begins.

999.M41: Last reported action of Ciaphas Cain (though his memoirs indicate he lives at least 30+ years beyond this point).

997.M41: Hive Fleet Leviathan invades the Galaxy, sparking the Third Tyrannic War which continues unabated to the 'present' of the setting. DAWN OF WAR II takes place at this time.

In short, it is somewhat implausible that the Blood Ravens would not know at all about the Tyranids, since their first invasion was 250 years earlier and Hive Fleet Leviathan had been invading the Galaxy for several years by that time. However, it is worth remembering that sectors and sub-sectors in the WH40K universe are vast, each consisting of millions of star systems and potentially hundreds of thousands of inhabitable planets, and are somewhat autonomous. The Tyranid threat had not previously affected the Blood Angels (their territory is remote from the Eastern Fringes, where all the previous incursions had taken place) and they are a notably single-minded chapter, even by Astartes standards. That which does not affect them they tend to disregard until it becomes relevant to them.