Blackavar is described as the black-fured rabbit from Efrafa. He tried to escape Efrafa, but was caught by Campion, his ears were torn and Blackavar was used as an example for every traitor that would try to excape. He is later liberated by Bigwig, and becomes a respected rabbit in Watership Down.
Unlike the movie, he does not die in a book and TV series.
(Blackavar in the miniseries)