The title refers to the black granite used by Germans for headstones of decorated soldiers and war memorials.

One can read "All Quiet on the Western Front" (in German, "Im Westen nichts Neues") and "Der Weg Zurueck" (The Road Back) as the prelude to the time period of The Black Obelisk. 

One of the best authors of the 20th century, IMO. His books were ordered burned when the Nazis came to power. Emigrated to the US in '39. The Black Obelisk was written in the 50's as I recall.